From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 29 Aug 2011 23:43:33 +0000 (-0400)
Subject: Print appropriate error on FUTEX_WAIT efault for RO shm
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=37ed587a8b9aea32c383058b4765a5730497d74a;p=ust.git

Print appropriate error on FUTEX_WAIT efault for RO shm

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

diff --git a/include/ust/usterr-signal-safe.h b/include/ust/usterr-signal-safe.h
index 483e7f5..71e7437 100644
--- a/include/ust/usterr-signal-safe.h
+++ b/include/ust/usterr-signal-safe.h
@@ -54,6 +54,8 @@ static inline int ust_debug(void)
 #define UST_XSTR(d) UST_STR(d)
 #define UST_STR(s) #s
 
+#define USTERR_MAX_LEN	512
+
 /* We sometimes print in the tracing path, and tracing can occur in
  * signal handlers, so we must use a print method which is signal safe.
  */
@@ -68,8 +70,8 @@ static inline void __attribute__ ((format (printf, 1, 2)))
 
 #define sigsafe_print_err(fmt, args...)					\
 {									\
-	/* Can't use dynamic allocation. Limit ourselves to 250 chars. */ \
-	char ____buf[250];						\
+	/* Can't use dynamic allocation. Limit ourselves to USTERR_MAX_LEN chars. */ \
+	char ____buf[USTERR_MAX_LEN];					\
 	int ____saved_errno;						\
 									\
 	/* Save the errno. */						\
diff --git a/libust/lttng-ust-comm.c b/libust/lttng-ust-comm.c
index a5f2b0b..2fd1657 100644
--- a/libust/lttng-ust-comm.c
+++ b/libust/lttng-ust-comm.c
@@ -119,6 +119,8 @@ struct sock_info local_apps = {
 	.socket = -1,
 };
 
+static int wait_poll_fallback;
+
 extern void ltt_ring_buffer_client_overwrite_init(void);
 extern void ltt_ring_buffer_client_discard_init(void);
 extern void ltt_ring_buffer_metadata_client_init(void);
@@ -485,6 +487,9 @@ void wait_for_sessiond(struct sock_info *sock_info)
 	if (lttng_ust_comm_should_quit) {
 		goto quit;
 	}
+	if (wait_poll_fallback) {
+		goto error;
+	}
 	if (!sock_info->wait_shm_mmap) {
 		sock_info->wait_shm_mmap = get_map_shm(sock_info);
 		if (!sock_info->wait_shm_mmap)
@@ -497,11 +502,16 @@ void wait_for_sessiond(struct sock_info *sock_info)
 	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);
-		/*
-		 * FIXME: Currently, futexes on read-only shm seems to
-		 * EFAULT.
-		 */
 		if (ret < 0) {
+			if (errno == EFAULT) {
+				wait_poll_fallback = 1;
+				ERR(
+"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.");
+			}
 			PERROR("futex");
 			sleep(5);
 		}