logging: Introduce mutex API to manage log_target in multi-thread envs

log_enable_multithread() enables use of locks inside the
implementation. Lock use is disabled by default, this way only
multi-thread processes need to enable it and suffer related
complexity/performance penalties.

Locks are required around osmo_log_target_list and items inside it,
since targets can be used, modified and deleted by different threads
concurrently (for instance, user writing "logging disable" in VTY while
another thread is willing to write into that target).

Multithread apps and libraries aiming at being used in multithread apps
should update their code to use the locks introduced here when
containing code iterating over osmo_log_target_list explictly or
implicitly by obtaining a log_target (eg. osmo_log_vty2tgt()).

Related: OS#4088
Change-Id: Id7711893b34263baacac6caf4d489467053131bb

12 files changed, 709 insertions, 100 deletions

diff --git a/configure.ac b/configure.ac
index 7ad5908d..39d232b3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -68,6 +68,9 @@ AC_SUBST(LIBRARY_DLSYM)
 AC_CHECK_LIB(execinfo, backtrace, BACKTRACE_LIB=-lexecinfo, BACKTRACE_LIB=)
 AC_SUBST(BACKTRACE_LIB)
 
+# check for pthread (PTHREAD_CFLAGS, PTHREAD_LIBS)
+AX_PTHREAD
+
 # check for old glibc < 2.17 to get clock_gettime
 AC_SEARCH_LIBS([clock_gettime], [rt posix4], [LIBRARY_RT="$LIBS";LIBS=""])
 AC_SUBST(LIBRARY_RT)
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 1a2d60bc..139d2916 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -380,4 +380,18 @@ void log_del_target(struct log_target *target);
 
 struct log_target *log_target_find(int type, const char *fname);
 
+void log_enable_multithread(void);
+
+void log_tgt_mutex_lock_impl(void);
+void log_tgt_mutex_unlock_impl(void);
+#define LOG_MTX_DEBUG 0
+#if LOG_MTX_DEBUG
+	#include <pthread.h>
+	#define log_tgt_mutex_lock() do { fprintf(stderr, "[%lu] %s:%d [%s] lock\n", pthread_self(), __FILE__, __LINE__, __func__); log_tgt_mutex_lock_impl(); } while (0)
+	#define log_tgt_mutex_unlock() do { fprintf(stderr, "[%lu] %s:%d [%s] unlock\n", pthread_self(), __FILE__, __LINE__, __func__); log_tgt_mutex_unlock_impl(); } while (0)
+#else
+	#define log_tgt_mutex_lock() log_tgt_mutex_lock_impl()
+	#define log_tgt_mutex_unlock() log_tgt_mutex_unlock_impl()
+#endif
+
 /*! @} */
diff --git a/libosmocore.pc.in b/libosmocore.pc.in
index d355659b..ce82d4f7 100644
--- a/libosmocore.pc.in
+++ b/libosmocore.pc.in
@@ -7,5 +7,5 @@ Name: Osmocom Core Library
 Description: C Utility Library
 Version: @VERSION@
 Libs: -L${libdir} @TALLOC_LIBS@ -losmocore
-Cflags: -I${includedir}/ @TALLOC_CFLAGS@
-
+Libs.private: @PTHREAD_LIBS@
+Cflags: -I${includedir}/ @TALLOC_CFLAGS@ @PTHREAD_CFLAGS@
diff --git a/m4/ax_pthread.m4 b/m4/ax_pthread.m4
new file mode 100644
index 00000000..4920e073
--- /dev/null
+++ b/m4/ax_pthread.m4
@@ -0,0 +1,486 @@
+# ===========================================================================
+#        https://www.gnu.org/software/autoconf-archive/ax_pthread.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PTHREAD([ACTION-IF-FOUND[, ACTION-IF-NOT-FOUND]])
+#
+# DESCRIPTION
+#
+#   This macro figures out how to build C programs using POSIX threads. It
+#   sets the PTHREAD_LIBS output variable to the threads library and linker
+#   flags, and the PTHREAD_CFLAGS output variable to any special C compiler
+#   flags that are needed. (The user can also force certain compiler
+#   flags/libs to be tested by setting these environment variables.)
+#
+#   Also sets PTHREAD_CC to any special C compiler that is needed for
+#   multi-threaded programs (defaults to the value of CC otherwise). (This
+#   is necessary on AIX to use the special cc_r compiler alias.)
+#
+#   NOTE: You are assumed to not only compile your program with these flags,
+#   but also to link with them as well. For example, you might link with
+#   $PTHREAD_CC $CFLAGS $PTHREAD_CFLAGS $LDFLAGS ... $PTHREAD_LIBS $LIBS
+#
+#   If you are only building threaded programs, you may wish to use these
+#   variables in your default LIBS, CFLAGS, and CC:
+#
+#     LIBS="$PTHREAD_LIBS $LIBS"
+#     CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+#     CC="$PTHREAD_CC"
+#
+#   In addition, if the PTHREAD_CREATE_JOINABLE thread-attribute constant
+#   has a nonstandard name, this macro defines PTHREAD_CREATE_JOINABLE to
+#   that name (e.g. PTHREAD_CREATE_UNDETACHED on AIX).
+#
+#   Also HAVE_PTHREAD_PRIO_INHERIT is defined if pthread is found and the
+#   PTHREAD_PRIO_INHERIT symbol is defined when compiling with
+#   PTHREAD_CFLAGS.
+#
+#   ACTION-IF-FOUND is a list of shell commands to run if a threads library
+#   is found, and ACTION-IF-NOT-FOUND is a list of commands to run it if it
+#   is not found. If ACTION-IF-FOUND is not specified, the default action
+#   will define HAVE_PTHREAD.
+#
+#   Please let the authors know if this macro fails on any platform, or if
+#   you have any other suggestions or comments. This macro was based on work
+#   by SGJ on autoconf scripts for FFTW (http://www.fftw.org/) (with help
+#   from M. Frigo), as well as ac_pthread and hb_pthread macros posted by
+#   Alejandro Forero Cuervo to the autoconf macro repository. We are also
+#   grateful for the helpful feedback of numerous users.
+#
+#   Updated for Autoconf 2.68 by Daniel Richard G.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Steven G. Johnson <stevenj@alum.mit.edu>
+#   Copyright (c) 2011 Daniel Richard G. <skunk@iSKUNK.ORG>
+#
+#   This program is free software: you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation, either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 25
+
+AU_ALIAS([ACX_PTHREAD], [AX_PTHREAD])
+AC_DEFUN([AX_PTHREAD], [
+AC_REQUIRE([AC_CANONICAL_HOST])
+AC_REQUIRE([AC_PROG_CC])
+AC_REQUIRE([AC_PROG_SED])
+AC_LANG_PUSH([C])
+ax_pthread_ok=no
+
+# We used to check for pthread.h first, but this fails if pthread.h
+# requires special compiler flags (e.g. on Tru64 or Sequent).
+# It gets checked for in the link test anyway.
+
+# First of all, check if the user has set any of the PTHREAD_LIBS,
+# etcetera environment variables, and if threads linking works using
+# them:
+if test "x$PTHREAD_CFLAGS$PTHREAD_LIBS" != "x"; then
+        ax_pthread_save_CC="$CC"
+        ax_pthread_save_CFLAGS="$CFLAGS"
+        ax_pthread_save_LIBS="$LIBS"
+        AS_IF([test "x$PTHREAD_CC" != "x"], [CC="$PTHREAD_CC"])
+        CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+        LIBS="$PTHREAD_LIBS $LIBS"
+        AC_MSG_CHECKING([for pthread_join using $CC $PTHREAD_CFLAGS $PTHREAD_LIBS])
+        AC_LINK_IFELSE([AC_LANG_CALL([], [pthread_join])], [ax_pthread_ok=yes])
+        AC_MSG_RESULT([$ax_pthread_ok])
+        if test "x$ax_pthread_ok" = "xno"; then
+                PTHREAD_LIBS=""
+                PTHREAD_CFLAGS=""
+        fi
+        CC="$ax_pthread_save_CC"
+        CFLAGS="$ax_pthread_save_CFLAGS"
+        LIBS="$ax_pthread_save_LIBS"
+fi
+
+# We must check for the threads library under a number of different
+# names; the ordering is very important because some systems
+# (e.g. DEC) have both -lpthread and -lpthreads, where one of the
+# libraries is broken (non-POSIX).
+
+# Create a list of thread flags to try.  Items starting with a "-" are
+# C compiler flags, and other items are library names, except for "none"
+# which indicates that we try without any flags at all, and "pthread-config"
+# which is a program returning the flags for the Pth emulation library.
+
+ax_pthread_flags="pthreads none -Kthread -pthread -pthreads -mthreads pthread --thread-safe -mt pthread-config"
+
+# The ordering *is* (sometimes) important.  Some notes on the
+# individual items follow:
+
+# pthreads: AIX (must check this before -lpthread)
+# none: in case threads are in libc; should be tried before -Kthread and
+#       other compiler flags to prevent continual compiler warnings
+# -Kthread: Sequent (threads in libc, but -Kthread needed for pthread.h)
+# -pthread: Linux/gcc (kernel threads), BSD/gcc (userland threads), Tru64
+#           (Note: HP C rejects this with "bad form for `-t' option")
+# -pthreads: Solaris/gcc (Note: HP C also rejects)
+# -mt: Sun Workshop C (may only link SunOS threads [-lthread], but it
+#      doesn't hurt to check since this sometimes defines pthreads and
+#      -D_REENTRANT too), HP C (must be checked before -lpthread, which
+#      is present but should not be used directly; and before -mthreads,
+#      because the compiler interprets this as "-mt" + "-hreads")
+# -mthreads: Mingw32/gcc, Lynx/gcc
+# pthread: Linux, etcetera
+# --thread-safe: KAI C++
+# pthread-config: use pthread-config program (for GNU Pth library)
+
+case $host_os in
+
+        freebsd*)
+
+        # -kthread: FreeBSD kernel threads (preferred to -pthread since SMP-able)
+        # lthread: LinuxThreads port on FreeBSD (also preferred to -pthread)
+
+        ax_pthread_flags="-kthread lthread $ax_pthread_flags"
+        ;;
+
+        hpux*)
+
+        # From the cc(1) man page: "[-mt] Sets various -D flags to enable
+        # multi-threading and also sets -lpthread."
+
+        ax_pthread_flags="-mt -pthread pthread $ax_pthread_flags"
+        ;;
+
+        openedition*)
+
+        # IBM z/OS requires a feature-test macro to be defined in order to
+        # enable POSIX threads at all, so give the user a hint if this is
+        # not set. (We don't define these ourselves, as they can affect
+        # other portions of the system API in unpredictable ways.)
+
+        AC_EGREP_CPP([AX_PTHREAD_ZOS_MISSING],
+            [
+#            if !defined(_OPEN_THREADS) && !defined(_UNIX03_THREADS)
+             AX_PTHREAD_ZOS_MISSING
+#            endif
+            ],
+            [AC_MSG_WARN([IBM z/OS requires -D_OPEN_THREADS or -D_UNIX03_THREADS to enable pthreads support.])])
+        ;;
+
+        solaris*)
+
+        # On Solaris (at least, for some versions), libc contains stubbed
+        # (non-functional) versions of the pthreads routines, so link-based
+        # tests will erroneously succeed. (N.B.: The stubs are missing
+        # pthread_cleanup_push, or rather a function called by this macro,
+        # so we could check for that, but who knows whether they'll stub
+        # that too in a future libc.)  So we'll check first for the
+        # standard Solaris way of linking pthreads (-mt -lpthread).
+
+        ax_pthread_flags="-mt,pthread pthread $ax_pthread_flags"
+        ;;
+esac
+
+# GCC generally uses -pthread, or -pthreads on some platforms (e.g. SPARC)
+
+AS_IF([test "x$GCC" = "xyes"],
+      [ax_pthread_flags="-pthread -pthreads $ax_pthread_flags"])
+
+# The presence of a feature test macro requesting re-entrant function
+# definitions is, on some systems, a strong hint that pthreads support is
+# correctly enabled
+
+case $host_os in
+        darwin* | hpux* | linux* | osf* | solaris*)
+        ax_pthread_check_macro="_REENTRANT"
+        ;;
+
+        aix*)
+        ax_pthread_check_macro="_THREAD_SAFE"
+        ;;
+
+        *)
+        ax_pthread_check_macro="--"
+        ;;
+esac
+AS_IF([test "x$ax_pthread_check_macro" = "x--"],
+      [ax_pthread_check_cond=0],
+      [ax_pthread_check_cond="!defined($ax_pthread_check_macro)"])
+
+# Are we compiling with Clang?
+
+AC_CACHE_CHECK([whether $CC is Clang],
+    [ax_cv_PTHREAD_CLANG],
+    [ax_cv_PTHREAD_CLANG=no
+     # Note that Autoconf sets GCC=yes for Clang as well as GCC
+     if test "x$GCC" = "xyes"; then
+        AC_EGREP_CPP([AX_PTHREAD_CC_IS_CLANG],
+            [/* Note: Clang 2.7 lacks __clang_[a-z]+__ */
+#            if defined(__clang__) && defined(__llvm__)
+             AX_PTHREAD_CC_IS_CLANG
+#            endif
+            ],
+            [ax_cv_PTHREAD_CLANG=yes])
+     fi
+    ])
+ax_pthread_clang="$ax_cv_PTHREAD_CLANG"
+
+ax_pthread_clang_warning=no
+
+# Clang needs special handling, because older versions handle the -pthread
+# option in a rather... idiosyncratic way
+
+if test "x$ax_pthread_clang" = "xyes"; then
+
+        # Clang takes -pthread; it has never supported any other flag
+
+        # (Note 1: This will need to be revisited if a system that Clang
+        # supports has POSIX threads in a separate library.  This tends not
+        # to be the way of modern systems, but it's conceivable.)
+
+        # (Note 2: On some systems, notably Darwin, -pthread is not needed
+        # to get POSIX threads support; the API is always present and
+        # active.  We could reasonably leave PTHREAD_CFLAGS empty.  But
+        # -pthread does define _REENTRANT, and while the Darwin headers
+        # ignore this macro, third-party headers might not.)
+
+        PTHREAD_CFLAGS="-pthread"
+        PTHREAD_LIBS=
+
+        ax_pthread_ok=yes
+
+        # However, older versions of Clang make a point of warning the user
+        # that, in an invocation where only linking and no compilation is
+        # taking place, the -pthread option has no effect ("argument unused
+        # during compilation").  They expect -pthread to be passed in only
+        # when source code is being compiled.
+        #
+        # Problem is, this is at odds with the way Automake and most other
+        # C build frameworks function, which is that the same flags used in
+        # compilation (CFLAGS) are also used in linking.  Many systems
+        # supported by AX_PTHREAD require exactly this for POSIX threads
+        # support, and in fact it is often not straightforward to specify a
+        # flag that is used only in the compilation phase and not in
+        # linking.  Such a scenario is extremely rare in practice.
+        #
+        # Even though use of the -pthread flag in linking would only print
+        # a warning, this can be a nuisance for well-run software projects
+        # that build with -Werror.  So if the active version of Clang has
+        # this misfeature, we search for an option to squash it.
+
+        AC_CACHE_CHECK([whether Clang needs flag to prevent "argument unused" warning when linking with -pthread],
+            [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG],
+            [ax_cv_PTHREAD_CLANG_NO_WARN_FLAG=unknown
+             # Create an alternate version of $ac_link that compiles and
+             # links in two steps (.c -> .o, .o -> exe) instead of one
+             # (.c -> exe), because the warning occurs only in the second
+             # step
+             ax_pthread_save_ac_link="$ac_link"
+             ax_pthread_sed='s/conftest\.\$ac_ext/conftest.$ac_objext/g'
+             ax_pthread_link_step=`$as_echo "$ac_link" | sed "$ax_pthread_sed"`
+             ax_pthread_2step_ac_link="($ac_compile) && (echo ==== >&5) && ($ax_pthread_link_step)"
+             ax_pthread_save_CFLAGS="$CFLAGS"
+             for ax_pthread_try in '' -Qunused-arguments -Wno-unused-command-line-argument unknown; do
+                AS_IF([test "x$ax_pthread_try" = "xunknown"], [break])
+                CFLAGS="-Werror -Wunknown-warning-option $ax_pthread_try -pthread $ax_pthread_save_CFLAGS"
+                ac_link="$ax_pthread_save_ac_link"
+                AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
+                    [ac_link="$ax_pthread_2step_ac_link"
+                     AC_LINK_IFELSE([AC_LANG_SOURCE([[int main(void){return 0;}]])],
+                         [break])
+                    ])
+             done
+             ac_link="$ax_pthread_save_ac_link"
+             CFLAGS="$ax_pthread_save_CFLAGS"
+             AS_IF([test "x$ax_pthread_try" = "x"], [ax_pthread_try=no])
+             ax_cv_PTHREAD_CLANG_NO_WARN_FLAG="$ax_pthread_try"
+            ])
+
+        case "$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG" in
+                no | unknown) ;;
+                *) PTHREAD_CFLAGS="$ax_cv_PTHREAD_CLANG_NO_WARN_FLAG $PTHREAD_CFLAGS" ;;
+        esac
+
+fi # $ax_pthread_clang = yes
+
+if test "x$ax_pthread_ok" = "xno"; then
+for ax_pthread_try_flag in $ax_pthread_flags; do
+
+        case $ax_pthread_try_flag in
+                none)
+                AC_MSG_CHECKING([whether pthreads work without any flags])
+                ;;
+
+                -mt,pthread)
+                AC_MSG_CHECKING([whether pthreads work with -mt -lpthread])
+                PTHREAD_CFLAGS="-mt"
+                PTHREAD_LIBS="-lpthread"
+                ;;
+
+                -*)
+                AC_MSG_CHECKING([whether pthreads work with $ax_pthread_try_flag])
+                PTHREAD_CFLAGS="$ax_pthread_try_flag"
+                ;;
+
+                pthread-config)
+                AC_CHECK_PROG([ax_pthread_config], [pthread-config], [yes], [no])
+                AS_IF([test "x$ax_pthread_config" = "xno"], [continue])
+                PTHREAD_CFLAGS="`pthread-config --cflags`"
+                PTHREAD_LIBS="`pthread-config --ldflags` `pthread-config --libs`"
+                ;;
+
+                *)
+                AC_MSG_CHECKING([for the pthreads library -l$ax_pthread_try_flag])
+                PTHREAD_LIBS="-l$ax_pthread_try_flag"
+                ;;
+        esac
+
+        ax_pthread_save_CFLAGS="$CFLAGS"
+        ax_pthread_save_LIBS="$LIBS"
+        CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+        LIBS="$PTHREAD_LIBS $LIBS"
+
+        # Check for various functions.  We must include pthread.h,
+        # since some functions may be macros.  (On the Sequent, we
+        # need a special flag -Kthread to make this header compile.)
+        # We check for pthread_join because it is in -lpthread on IRIX
+        # while pthread_create is in libc.  We check for pthread_attr_init
+        # due to DEC craziness with -lpthreads.  We check for
+        # pthread_cleanup_push because it is one of the few pthread
+        # functions on Solaris that doesn't have a non-functional libc stub.
+        # We try pthread_create on general principles.
+
+        AC_LINK_IFELSE([AC_LANG_PROGRAM([#include <pthread.h>
+#                       if $ax_pthread_check_cond
+#                        error "$ax_pthread_check_macro must be defined"
+#                       endif
+                        static void routine(void *a) { a = 0; }
+                        static void *start_routine(void *a) { return a; }],
+                       [pthread_t th; pthread_attr_t attr;
+                        pthread_create(&th, 0, start_routine, 0);
+                        pthread_join(th, 0);
+                        pthread_attr_init(&attr);
+                        pthread_cleanup_push(routine, 0);
+                        pthread_cleanup_pop(0) /* ; */])],
+            [ax_pthread_ok=yes],
+            [])
+
+        CFLAGS="$ax_pthread_save_CFLAGS"
+        LIBS="$ax_pthread_save_LIBS"
+
+        AC_MSG_RESULT([$ax_pthread_ok])
+        AS_IF([test "x$ax_pthread_ok" = "xyes"], [break])
+
+        PTHREAD_LIBS=""
+        PTHREAD_CFLAGS=""
+done
+fi
+
+# Various other checks:
+if test "x$ax_pthread_ok" = "xyes"; then
+        ax_pthread_save_CFLAGS="$CFLAGS"
+        ax_pthread_save_LIBS="$LIBS"
+        CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+        LIBS="$PTHREAD_LIBS $LIBS"
+
+        # Detect AIX lossage: JOINABLE attribute is called UNDETACHED.
+        AC_CACHE_CHECK([for joinable pthread attribute],
+            [ax_cv_PTHREAD_JOINABLE_ATTR],
+            [ax_cv_PTHREAD_JOINABLE_ATTR=unknown
+             for ax_pthread_attr in PTHREAD_CREATE_JOINABLE PTHREAD_CREATE_UNDETACHED; do
+                 AC_LINK_IFELSE([AC_LANG_PROGRAM([#include <pthread.h>],
+                                                 [int attr = $ax_pthread_attr; return attr /* ; */])],
+                                [ax_cv_PTHREAD_JOINABLE_ATTR=$ax_pthread_attr; break],
+                                [])
+             done
+            ])
+        AS_IF([test "x$ax_cv_PTHREAD_JOINABLE_ATTR" != "xunknown" && \
+               test "x$ax_cv_PTHREAD_JOINABLE_ATTR" != "xPTHREAD_CREATE_JOINABLE" && \
+               test "x$ax_pthread_joinable_attr_defined" != "xyes"],
+              [AC_DEFINE_UNQUOTED([PTHREAD_CREATE_JOINABLE],
+                                  [$ax_cv_PTHREAD_JOINABLE_ATTR],
+                                  [Define to necessary symbol if this constant
+                                   uses a non-standard name on your system.])
+               ax_pthread_joinable_attr_defined=yes
+              ])
+
+        AC_CACHE_CHECK([whether more special flags are required for pthreads],
+            [ax_cv_PTHREAD_SPECIAL_FLAGS],
+            [ax_cv_PTHREAD_SPECIAL_FLAGS=no
+             case $host_os in
+             solaris*)
+             ax_cv_PTHREAD_SPECIAL_FLAGS="-D_POSIX_PTHREAD_SEMANTICS"
+             ;;
+             esac
+            ])
+        AS_IF([test "x$ax_cv_PTHREAD_SPECIAL_FLAGS" != "xno" && \
+               test "x$ax_pthread_special_flags_added" != "xyes"],
+              [PTHREAD_CFLAGS="$ax_cv_PTHREAD_SPECIAL_FLAGS $PTHREAD_CFLAGS"
+               ax_pthread_special_flags_added=yes])
+
+        AC_CACHE_CHECK([for PTHREAD_PRIO_INHERIT],
+            [ax_cv_PTHREAD_PRIO_INHERIT],
+            [AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <pthread.h>]],
+                                             [[int i = PTHREAD_PRIO_INHERIT;
+                                               return i;]])],
+                            [ax_cv_PTHREAD_PRIO_INHERIT=yes],
+                            [ax_cv_PTHREAD_PRIO_INHERIT=no])
+            ])
+        AS_IF([test "x$ax_cv_PTHREAD_PRIO_INHERIT" = "xyes" && \
+               test "x$ax_pthread_prio_inherit_defined" != "xyes"],
+              [AC_DEFINE([HAVE_PTHREAD_PRIO_INHERIT], [1], [Have PTHREAD_PRIO_INHERIT.])
+               ax_pthread_prio_inherit_defined=yes
+              ])
+
+        CFLAGS="$ax_pthread_save_CFLAGS"
+        LIBS="$ax_pthread_save_LIBS"
+
+        # More AIX lossage: compile with *_r variant
+        if test "x$GCC" != "xyes"; then
+            case $host_os in
+                aix*)
+                AS_CASE(["x/$CC"],
+                    [x*/c89|x*/c89_128|x*/c99|x*/c99_128|x*/cc|x*/cc128|x*/xlc|x*/xlc_v6|x*/xlc128|x*/xlc128_v6],
+                    [#handle absolute path differently from PATH based program lookup
+                     AS_CASE(["x$CC"],
+                         [x/*],
+                         [AS_IF([AS_EXECUTABLE_P([${CC}_r])],[PTHREAD_CC="${CC}_r"])],
+                         [AC_CHECK_PROGS([PTHREAD_CC],[${CC}_r],[$CC])])])
+                ;;
+            esac
+        fi
+fi
+
+test -n "$PTHREAD_CC" || PTHREAD_CC="$CC"
+
+AC_SUBST([PTHREAD_LIBS])
+AC_SUBST([PTHREAD_CFLAGS])
+AC_SUBST([PTHREAD_CC])
+
+# Finally, execute ACTION-IF-FOUND/ACTION-IF-NOT-FOUND:
+if test "x$ax_pthread_ok" = "xyes"; then
+        ifelse([$1],,[AC_DEFINE([HAVE_PTHREAD],[1],[Define if you have POSIX threads libraries and header files.])],[$1])
+        :
+else
+        ax_pthread_ok=no
+        $2
+fi
+AC_LANG_POP
+])dnl AX_PTHREAD
diff --git a/src/Makefile.am b/src/Makefile.am
index 245eb6d5..5f5f0171 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -4,7 +4,7 @@
 LIBVERSION=14:0:2
 
 AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_builddir)/include
-AM_CFLAGS = -Wall $(TALLOC_CFLAGS)
+AM_CFLAGS = -Wall $(TALLOC_CFLAGS) $(PTHREAD_CFLAGS)
 
 if ENABLE_PSEUDOTALLOC
 AM_CPPFLAGS += -I$(top_srcdir)/src/pseudotalloc
@@ -12,7 +12,7 @@ endif
 
 lib_LTLIBRARIES = libosmocore.la
 
-libosmocore_la_LIBADD = $(BACKTRACE_LIB) $(TALLOC_LIBS) $(LIBRARY_RT)
+libosmocore_la_LIBADD = $(BACKTRACE_LIB) $(TALLOC_LIBS) $(LIBRARY_RT) $(PTHREAD_LIBS)
 libosmocore_la_SOURCES = context.c timer.c timer_gettimeofday.c timer_clockgettime.c \
 			 select.c signal.c msgb.c bits.c \
 			 bitvec.c bitcomp.c counter.c fsm.c \
diff --git a/src/gb/gprs_bssgp_vty.c b/src/gb/gprs_bssgp_vty.c
index 3af6517f..5dab94e7 100644
--- a/src/gb/gprs_bssgp_vty.c
+++ b/src/gb/gprs_bssgp_vty.c
@@ -181,21 +181,27 @@ DEFUN(logging_fltr_bvc,
 	"BVCI of the BVC to be filtered\n"
 	"BSSGP Virtual Connection Identifier (BVCI)\n")
 {
-	struct log_target *tgt = osmo_log_vty2tgt(vty);
+	struct log_target *tgt;
 	struct bssgp_bvc_ctx *bvc;
 	uint16_t nsei = atoi(argv[0]);
 	uint16_t bvci = atoi(argv[1]);
 
-	if (!tgt)
+	log_tgt_mutex_lock();
+	tgt = osmo_log_vty2tgt(vty);
+	if (!tgt) {
+		log_tgt_mutex_unlock();
 		return CMD_WARNING;
+	}
 
 	bvc = btsctx_by_bvci_nsei(bvci, nsei);
 	if (!bvc) {
 		vty_out(vty, "No BVC by that identifier%s", VTY_NEWLINE);
+		log_tgt_mutex_unlock();
 		return CMD_WARNING;
 	}
 
 	log_set_bvc_filter(tgt, bvc);
+	log_tgt_mutex_unlock();
 	return CMD_SUCCESS;
 }
 
diff --git a/src/gb/gprs_ns_vty.c b/src/gb/gprs_ns_vty.c
index 53c71a9a..4a904368 100644
--- a/src/gb/gprs_ns_vty.c
+++ b/src/gb/gprs_ns_vty.c
@@ -587,12 +587,16 @@ DEFUN(logging_fltr_nsvc,
 	"Identify NS-VC by NSVCI\n"
 	"Numeric identifier\n")
 {
-	struct log_target *tgt = osmo_log_vty2tgt(vty);
+	struct log_target *tgt;
 	struct gprs_nsvc *nsvc;
 	uint16_t id = atoi(argv[1]);
 
-	if (!tgt)
+	log_tgt_mutex_lock();
+	tgt = osmo_log_vty2tgt(vty);
+	if (!tgt) {
+		log_tgt_mutex_unlock();
 		return CMD_WARNING;
+	}
 
 	if (!strcmp(argv[0], "nsei"))
 		nsvc = gprs_nsvc_by_nsei(vty_nsi, id);
@@ -601,10 +605,12 @@ DEFUN(logging_fltr_nsvc,
 
 	if (!nsvc) {
 		vty_out(vty, "No NS-VC by that identifier%s", VTY_NEWLINE);
+		log_tgt_mutex_unlock();
 		return CMD_WARNING;
 	}
 
 	log_set_nsvc_filter(tgt, nsvc);
+	log_tgt_mutex_unlock();
 	return CMD_SUCCESS;
 }
 
diff --git a/src/logging.c b/src/logging.c
index 1c3544fa..b030f8a6 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -42,6 +42,7 @@
 #include <time.h>
 #include <sys/time.h>
 #include <errno.h>
+#include <pthread.h>
 
 #include <osmocom/core/talloc.h>
 #include <osmocom/core/utils.h>
@@ -63,6 +64,56 @@ static struct log_context log_context;
 void *tall_log_ctx = NULL;
 LLIST_HEAD(osmo_log_target_list);
 
+#if (!EMBEDDED)
+/*! This mutex must be held while using osmo_log_target_list or any of its
+  log_targets in a multithread program. Prevents race conditions between threads
+  like producing unordered timestamps or VTY deleting a target while another
+  thread is writing to it */
+static pthread_mutex_t osmo_log_tgt_mutex;
+static bool osmo_log_tgt_mutex_on = false;
+
+/*! Enable multithread support (mutex) in libosmocore logging system.
+ * Must be called by processes willing to use logging subsystem from several
+ * threads. Once enabled, it's not possible to disable it again.
+ */
+void log_enable_multithread(void) {
+	if (osmo_log_tgt_mutex_on)
+		return;
+	pthread_mutex_init(&osmo_log_tgt_mutex, NULL);
+	osmo_log_tgt_mutex_on = true;
+}
+
+/*! Acquire the osmo_log_tgt_mutex. Don't use this function directly, always use
+ *  macro log_tgt_mutex_lock() instead.
+ */
+void log_tgt_mutex_lock_impl(void) {
+	/* These lines are useful to debug scenarios where there's only 1 thread
+	   and a double lock appears, for instance during startup and some
+	   unlock() missing somewhere:
+	if (osmo_log_tgt_mutex_on && pthread_mutex_trylock(&osmo_log_tgt_mutex) != 0)
+		osmo_panic("acquiring already locked mutex!\n");
+	return;
+	*/
+
+	if (osmo_log_tgt_mutex_on)
+		pthread_mutex_lock(&osmo_log_tgt_mutex);
+}
+
+/*! Release the osmo_log_tgt_mutex. Don't use this function directly, always use
+ *  macro log_tgt_mutex_unlock() instead.
+ */
+void log_tgt_mutex_unlock_impl(void) {
+	if (osmo_log_tgt_mutex_on)
+		pthread_mutex_unlock(&osmo_log_tgt_mutex);
+}
+
+#else /* if (!EMBEDDED) */
+#pragma message ("logging multithread support disabled in embedded build")
+void log_enable_multithread(void) {}
+void log_tgt_mutex_lock_impl(void) {}
+void log_tgt_mutex_unlock_impl(void) {}
+#endif /* if (!EMBEDDED) */
+
 const struct value_string loglevel_strs[] = {
 	{ LOGL_DEBUG,	"DEBUG" },
 	{ LOGL_INFO,	"INFO" },
@@ -532,6 +583,8 @@ void osmo_vlogp(int subsys, int level, const char *file, int line,
 
 	subsys = map_subsys(subsys);
 
+	log_tgt_mutex_lock();
+
 	llist_for_each_entry(tar, &osmo_log_target_list, entry) {
 		va_list bp;
 
@@ -548,6 +601,8 @@ void osmo_vlogp(int subsys, int level, const char *file, int line,
 			_output(tar, subsys, level, file, line, cont, format, bp);
 		va_end(bp);
 	}
+
+	log_tgt_mutex_unlock();
 }
 
 /*! logging function used by DEBUGP() macro
@@ -870,6 +925,7 @@ struct log_target *log_target_create_file(const char *fname)
  *  \param[in] type Log target type
  *  \param[in] fname File name
  *  \returns Log target (if found), NULL otherwise
+ *  Must be called with mutex osmo_log_tgt_mutex held, see log_tgt_mutex_lock.
  */
 struct log_target *log_target_find(int type, const char *fname)
 {
@@ -942,6 +998,8 @@ int log_targets_reopen(void)
 	struct log_target *tar;
 	int rc = 0;
 
+	log_tgt_mutex_lock();
+
 	llist_for_each_entry(tar, &osmo_log_target_list, entry) {
 		switch (tar->type) {
 		case LOG_TGT_TYPE_FILE:
@@ -953,6 +1011,8 @@ int log_targets_reopen(void)
 		}
 	}
 
+	log_tgt_mutex_unlock();
+
 	return rc;
 }
 
@@ -1015,6 +1075,8 @@ void log_fini(void)
 {
 	struct log_target *tar, *tar2;
 
+	log_tgt_mutex_lock();
+
 	llist_for_each_entry_safe(tar, tar2, &osmo_log_target_list, entry)
 		log_target_destroy(tar);
 
@@ -1022,6 +1084,8 @@ void log_fini(void)
 	osmo_log_info = NULL;
 	talloc_free(tall_log_ctx);
 	tall_log_ctx = NULL;
+
+	log_tgt_mutex_unlock();
 }
 
 /*! Check whether a log entry will be generated.
@@ -1036,15 +1100,19 @@ int log_check_level(int subsys, unsigned int level)
 
 	/* TODO: The following could/should be cached (update on config) */
 
+	log_tgt_mutex_lock();
+
 	llist_for_each_entry(tar, &osmo_log_target_list, entry) {
 		if (!should_log_to_target(tar, subsys, level))
 			continue;
 
 		/* This might get logged (ignoring filters) */
+		log_tgt_mutex_unlock();
 		return 1;
 	}
 
 	/* We are sure, that this will not be logged. */
+	log_tgt_mutex_unlock();
 	return 0;
 }
 
diff --git a/src/vty/logging_vty.c b/src/vty/logging_vty.c
index d639a8f7..88ee330a 100644
--- a/src/vty/logging_vty.c
+++ b/src/vty/logging_vty.c
@@ -101,6 +101,25 @@ struct log_target *log_target_create_vty(struct vty *vty)
 	return target;
 }
 
+/*! Get tgt with log lock acquired, return and release lock with warning if tgt
+ *  is not found. Lock must be released later with log_tgt_mutex_unlock().
+ */
+#define ACQUIRE_VTY_LOG_TGT_WITH_LOCK(vty, tgt) \
+	do { \
+		log_tgt_mutex_lock(); \
+		tgt = osmo_log_vty2tgt(vty); \
+		if (!(tgt)) { \
+			log_tgt_mutex_unlock(); \
+			return CMD_WARNING; \
+		} \
+	} while (0)
+
+#define RET_WITH_UNLOCK(ret) \
+	do { \
+		log_tgt_mutex_unlock(); \
+		return (ret); \
+	} while (0)
+
 DEFUN(enable_logging,
       enable_logging_cmd,
       "logging enable",
@@ -118,11 +137,16 @@ DEFUN(enable_logging,
 	conn->dbg = log_target_create_vty(vty);
 	if (!conn->dbg)
 		return CMD_WARNING;
-
+	log_tgt_mutex_lock();
 	log_add_target(conn->dbg);
-	return CMD_SUCCESS;
+	RET_WITH_UNLOCK(CMD_SUCCESS);
 }
 
+/*! Get log target associated to VTY console.
+ *  \param[in] vty Log target type
+ *  \returns Log target (if logging enabled), NULL otherwise
+ *  Must be called with mutex osmo_log_tgt_mutex held, see log_tgt_mutex_lock.
+ */
 struct log_target *osmo_log_vty2tgt(struct vty *vty)
 {
 	struct telnet_connection *conn;
@@ -146,13 +170,12 @@ DEFUN(logging_fltr_all,
 	"Only print messages matched by other filters\n"
 	"Bypass filter and print all messages\n")
 {
-	struct log_target *tgt = osmo_log_vty2tgt(vty);
+	struct log_target *tgt;
 
-	if (!tgt)
-		return CMD_WARNING;
+	ACQUIRE_VTY_LOG_TGT_WITH_LOCK(vty, tgt);
 
 	log_set_all_filter(tgt, atoi(argv[0]));
-	return CMD_SUCCESS;
+	RET_WITH_UNLOCK(CMD_SUCCESS);
 }
 
 DEFUN(logging_use_clr,
@@ -162,13 +185,12 @@ DEFUN(logging_use_clr,
       "Don't use color for printing messages\n"
       "Use color for printing messages\n")
 {
-	struct log_target *tgt = osmo_log_vty2tgt(vty);
+	struct log_target *tgt;
 
-	if (!tgt)
-		return CMD_WARNING;
+	ACQUIRE_VTY_LOG_TGT_WITH_LOCK(vty, tgt);
 
 	log_set_use_color(tgt, atoi(argv[0]));
-	return CMD_SUCCESS;
+	RET_WITH_UNLOCK(CMD_SUCCESS);
 }
 
 DEFUN(logging_prnt_timestamp,
@@ -178,13 +200,12 @@ DEFUN(logging_prnt_timestamp,
 	"Don't prefix each log message\n"
 	"Prefix each log message with current timestamp\n")
 {
-	struct log_target *tgt = osmo_log_vty2tgt(vty);
+	struct log_target *tgt;
 
-	if (!tgt)
-		return CMD_WARNING;
+	ACQUIRE_VTY_LOG_TGT_WITH_LOCK(vty, tgt);
 
 	log_set_print_timestamp(tgt, atoi(argv[0]));
-	return CMD_SUCCESS;
+	RET_WITH_UNLOCK(CMD_SUCCESS);
 }
 
 DEFUN(logging_prnt_ext_timestamp,
@@ -195,13 +216,12 @@ DEFUN(logging_prnt_ext_timestamp,
 	"Don't prefix each log message\n"
 	"Prefix each log message with current timestamp with YYYYMMDDhhmmssnnn\n")