Document ust lock async-signal-safety

[lttng-ust.git] / src / lib / lttng-ust / lttng-ust-comm.c

diff --git a/src/lib/lttng-ust/lttng-ust-comm.c b/src/lib/lttng-ust/lttng-ust-comm.c
index d67550797c5443aef3d6342db83c612450458cdb..10883e9675b6374f42bfa38d0b7529eb8cbc05e1 100644 (file)
--- a/src/lib/lttng-ust/lttng-ust-comm.c
+++ b/src/lib/lttng-ust/lttng-ust-comm.c
@@ -1,7 +1,7 @@
 /*
  * SPDX-License-Identifier: LGPL-2.1-only
  *
- * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
+ * Copyright (C) 2011 EfficiOS Inc.
  * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  */
 
@@ -118,6 +118,28 @@ static int lttng_ust_comm_should_quit;
  */
 int lttng_ust_loaded __attribute__((weak));
 
+/*
+ * Notes on async-signal-safety of ust lock: a few libc functions are used
+ * which are not strictly async-signal-safe:
+ *
+ * - pthread_setcancelstate
+ * - pthread_mutex_lock
+ * - pthread_mutex_unlock
+ *
+ * As of glibc 2.35, the implementation of pthread_setcancelstate only
+ * touches TLS data, and it appears to be safe to use from signal
+ * handlers. If the libc implementation changes, this will need to be
+ * revisited, and we may ask glibc to provide an async-signal-safe
+ * pthread_setcancelstate.
+ *
+ * As of glibc 2.35, the implementation of pthread_mutex_lock/unlock
+ * for fast mutexes only relies on the pthread_mutex_t structure.
+ * Disabling signals around all uses of this mutex ensures
+ * signal-safety. If the libc implementation changes and eventually uses
+ * other global resources, this will need to be revisited and we may
+ * need to implement our own mutex.
+ */
+
 /*
  * Return 0 on success, -1 if should quit.
  * The lock is taken in both cases.
@@ -134,13 +156,13 @@ int ust_lock(void)
        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_mask);
        if (ret) {
-               ERR("pthread_sigmask: %s", strerror(ret));
+               ERR("pthread_sigmask: ret=%d", ret);
        }
        if (!URCU_TLS(ust_mutex_nest)++)
                pthread_mutex_lock(&ust_mutex);
        ret = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (ret) {
-               ERR("pthread_sigmask: %s", strerror(ret));
+               ERR("pthread_sigmask: ret=%d", ret);
        }
        if (lttng_ust_comm_should_quit) {
                return -1;
@@ -166,13 +188,13 @@ void ust_lock_nocheck(void)
        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_mask);
        if (ret) {
-               ERR("pthread_sigmask: %s", strerror(ret));
+               ERR("pthread_sigmask: ret=%d", ret);
        }
        if (!URCU_TLS(ust_mutex_nest)++)
                pthread_mutex_lock(&ust_mutex);
        ret = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (ret) {
-               ERR("pthread_sigmask: %s", strerror(ret));
+               ERR("pthread_sigmask: ret=%d", ret);
        }
 }
 
@@ -187,13 +209,13 @@ void ust_unlock(void)
        sigfillset(&sig_all_blocked);
        ret = pthread_sigmask(SIG_SETMASK, &sig_all_blocked, &orig_mask);
        if (ret) {
-               ERR("pthread_sigmask: %s", strerror(ret));
+               ERR("pthread_sigmask: ret=%d", ret);
        }
        if (!--URCU_TLS(ust_mutex_nest))
                pthread_mutex_unlock(&ust_mutex);
        ret = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (ret) {
-               ERR("pthread_sigmask: %s", strerror(ret));
+               ERR("pthread_sigmask: ret=%d", ret);
        }
        if (lttng_ust_cancelstate_disable_pop()) {
                ERR("lttng_ust_cancelstate_disable_pop");