On Thu, 2011-05-19 at 20:21 -0300, Leonardo Chiquitto wrote:
> 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:
This looks like a rather interesting mistake on my part.
It doesn't look simple to fix either, anyway I'm working on it.
>
> (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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