Fix: handle sys_futex EINTR and EWOULDBLOCK

Handling sys_futex EINTR allows us to retry waiting on the futex
immediately if we are interrupted by a signal without having to try
connecting to the session daemon.

This should not impact correctness though, as the wait scheme would
simply try to connect, fail, and try waiting again.

Also handle errors on FUTEX_WAKE.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
index b290204024afcd4a761778cb5e359d42501a9ba5..7c746d8a712a2d30c271a016d92bec53673fb971 100644 (file)
--- a/liblttng-ust/lttng-ust-comm.c
+++ b/liblttng-ust/lttng-ust-comm.c
@@ -1120,8 +1120,6 @@ error:
 static
 void wait_for_sessiond(struct sock_info *sock_info)
 {
-       int ret;
-
        if (ust_lock()) {
                goto quit;
        }
@@ -1137,23 +1135,32 @@ void wait_for_sessiond(struct sock_info *sock_info)
 
        DBG("Waiting for %s apps sessiond", sock_info->name);
        /* Wait for futex wakeup */
-       if (uatomic_read((int32_t *) sock_info->wait_shm_mmap) == 0) {
-               ret = futex_async((int32_t *) sock_info->wait_shm_mmap,
-                       FUTEX_WAIT, 0, NULL, NULL, 0);
-               if (ret < 0) {
-                       if (errno == EFAULT) {
-                               wait_poll_fallback = 1;
-                               DBG(
+       if (uatomic_read((int32_t *) sock_info->wait_shm_mmap))
+               goto end_wait;
+
+       while (futex_async((int32_t *) sock_info->wait_shm_mmap,
+                       FUTEX_WAIT, 0, NULL, NULL, 0)) {
+               switch (errno) {
+               case EWOULDBLOCK:
+                       /* Value already changed. */
+                       goto end_wait;
+               case EINTR:
+                       /* Retry if interrupted by signal. */
+                       break;  /* Get out of switch. */
+               case EFAULT:
+                       wait_poll_fallback = 1;
+                       DBG(
 "Linux kernels 2.6.33 to 3.0 (with the exception of stable versions) "
 "do not support FUTEX_WAKE on read-only memory mappings correctly. "
 "Please upgrade your kernel "
 "(fix is commit 9ea71503a8ed9184d2d0b8ccc4d269d05f7940ae in Linux kernel "
 "mainline). LTTng-UST will use polling mode fallback.");
-                               if (ust_debug())
-                                       PERROR("futex");
-                       }
+                       if (ust_debug())
+                               PERROR("futex");
+                       goto end_wait;
                }
        }
+end_wait:
        return;
 
 quit:

diff --git a/tests/ust-basic-tracing/ust-basic-tracing.c b/tests/ust-basic-tracing/ust-basic-tracing.c
index 7bfeb5bc540f7037a15a8a61976f0a2ebbfcc2ef..cc0fb89b65721fb9fb4329cdae07e2c23b74a604 100644 (file)
--- a/tests/ust-basic-tracing/ust-basic-tracing.c
+++ b/tests/ust-basic-tracing/ust-basic-tracing.c
@@ -659,8 +659,11 @@ int update_futex(int fd, int active)
 
        if (active) {
                uatomic_set((int32_t *) wait_shm_mmap, 1);
-               futex_async((int32_t *) wait_shm_mmap, FUTEX_WAKE,
-                               INT_MAX, NULL, NULL, 0);
+               if (futex_async((int32_t *) wait_shm_mmap, FUTEX_WAKE,
+                               INT_MAX, NULL, NULL, 0) < 0) {
+                       perror("futex_async");
+                       goto error;
+               }
        } else {
                uatomic_set((int32_t *) wait_shm_mmap, 0);
        }

diff --git a/tests/ust-multi-test/ust-multi-test.c b/tests/ust-multi-test/ust-multi-test.c
index 7771154014376b79b92ae063601355fd3113ee51..c003c5a93f0f00bf033be2a54cccd7015b7996f5 100644 (file)
--- a/tests/ust-multi-test/ust-multi-test.c
+++ b/tests/ust-multi-test/ust-multi-test.c
@@ -655,8 +655,11 @@ int update_futex(int fd, int active)
 
        if (active) {
                uatomic_set((int32_t *) wait_shm_mmap, 1);
-               futex_async((int32_t *) wait_shm_mmap, FUTEX_WAKE,
-                               INT_MAX, NULL, NULL, 0);
+               if (futex_async((int32_t *) wait_shm_mmap, FUTEX_WAKE,
+                               INT_MAX, NULL, NULL, 0) < 0) {
+                       perror("futex_async");
+                       goto error;
+               }
        } else {
                uatomic_set((int32_t *) wait_shm_mmap, 0);
        }