Hello,
We received a support request from a customer reporting a hang in the
automount daemon. Analyzing the core dump, it looks like automount
can deadlock if two threads execute in the following order:
(All the line numbers are adjusted to match the latest version from Git)
Thread 7 (Thread 25043): Thread 6 (Thread 24007):
#9 start_thread() libpthread.so.0 #3 start_thread()
libpthread.so.0
#8 do_read_master() at automount.c:1259 #2 do_readmap() at state.c:479
#7 master_read_master() at master.c:853 --> master_mutex_lock()
[state.c:462]
--> master_mutex_unlock()
[state.c:466]
--> master_mutex_lock() [master.c:836]
--> cache_writelock() [master.c:838 or :849]
-->
master_source_readlock() [state.c:477]
#6 lookup_nss_read_master() at lookup.c:229
#5 do_read_master() at lookup.c:96
#4 lookup_read_master() at lookup_ldap.c:1676
#3 master_parse_entry() at master_parse.y:829
#2 master_add_map_source() at master.c:192
#1 master_source_writelock() at master.c:543 #1 cache_readlock() at
cache.c:60
#0 pthread_rwlock_wrlock() libpthread.so.0 #0 pthread_rwlock_rdlock()
libpthread.so.0
At this point:
Thread 7: locked(master_mutex_lock)
locked(cache_writelock) A
waits(master_source_writelock) B
Thread 6: locked(master_source_readlock) B
waits(cache_readlock) A
The AutoFS version is 5.0.5 plus all upstream patches up to
autofs-5.0.5-fix-submount-shutdown-wait.patch plus
autofs-5.0.5-fix-out-of-order-locking-in-readmap.patch
autofs-5.0.5-fix-next-task-list-update.patch
autofs-5.0.5-fix-stale-map-read.patch
At the first sight, moving pthread_cleanup_pop(1) (and consequently
master_mutex_unlock()) to the end of do_readmap() could avoid the
problem, but I didn't test this yet (please see untested patch below).
Any insight would be much appreciated!
Thanks,
Leonardo
Index: autofs/daemon/state.c
===================================================================
--- autofs.orig/daemon/state.c
+++ autofs/daemon/state.c
@@ -463,7 +463,6 @@ static void *do_readmap(void *arg)
status = lookup_nss_read_map(ap, NULL, now);
if (!status)
pthread_exit(NULL);
- pthread_cleanup_pop(1);
if (ap->type == LKP_INDIRECT) {
lookup_prune_cache(ap, now);
@@ -504,6 +503,7 @@ static void *do_readmap(void *arg)
}
pthread_cleanup_pop(1);
+ pthread_cleanup_pop(1);
return NULL;
}
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