Hello,
Hit this lockdep warning today (with 3.2-merge-window kernel). Below is my
naive approach.
------8<-------8<--------
Fix possible recursive locking, because both sys_epoll_ctl() and ep_scan_ready_list()
were trying to acquire the same ep->mtx mutex.
[ 23.563357] systemd-logind/2373 is trying to acquire lock:
[ 23.563568] (&ep->mtx){+.+.+.}, at: [<ffffffff81146da5>] ep_scan_ready_list+0x3c/0x1a7
[ 23.563922]
[ 23.563924] but task is already holding lock:
[ 23.564155] (&ep->mtx){+.+.+.}, at: [<ffffffff81147614>] sys_epoll_ctl+0x11e/0x20b
[ 23.564504]
[ 23.565126] 2 locks held by systemd-logind/2373:
[ 23.565130] #0: (epmutex){+.+.+.}, at: [<ffffffff811475cb>] sys_epoll_ctl+0xd5/0x20b
[ 23.565152] #1: (&ep->mtx){+.+.+.}, at: [<ffffffff81147614>] sys_epoll_ctl+0x11e/0x20b
[ 23.565165]
[ 23.565166] stack backtrace:
[ 23.565174] Pid: 2373, comm: systemd-logind Not tainted 3.1.0-dbg-05709-g189f65b-dirty #759
[ 23.565183] Call Trace:
[ 23.565199] [<ffffffff81072566>] print_deadlock_bug+0xea/0xf5
[ 23.565210] [<ffffffff81072696>] check_deadlock.isra.26+0x125/0x149
[ 23.565220] [<ffffffff81073af5>] validate_chain.isra.30+0x445/0x44f
[ 23.565229] [<ffffffff810746fd>] __lock_acquire+0x67e/0x6f5
[ 23.565244] [<ffffffff81146da5>] ? ep_scan_ready_list+0x3c/0x1a7
[ 23.565252] [<ffffffff81074d08>] lock_acquire+0x138/0x1b3
[ 23.565259] [<ffffffff81146da5>] ? ep_scan_ready_list+0x3c/0x1a7
[ 23.565268] [<ffffffff81146da5>] ? ep_scan_ready_list+0x3c/0x1a7
[ 23.565287] [<ffffffff814a4399>] mutex_lock_nested+0x5e/0x325
[ 23.565296] [<ffffffff81146da5>] ? ep_scan_ready_list+0x3c/0x1a7
[ 23.565307] [<ffffffff81146624>] ? sys_fanotify_mark+0x246/0x246
[ 23.565314] [<ffffffff81146da5>] ep_scan_ready_list+0x3c/0x1a7
[ 23.565322] [<ffffffff81146f10>] ? ep_scan_ready_list+0x1a7/0x1a7
[ 23.565332] [<ffffffff81146f22>] ep_poll_readyevents_proc+0x12/0x14
[ 23.565340] [<ffffffff81146b0a>] ep_call_nested.constprop.5+0x95/0xe1
[ 23.565352] [<ffffffff81146c35>] ep_eventpoll_poll+0x49/0x57
[ 23.565363] [<ffffffff814970c2>] ep_insert+0xd5/0x282
[ 23.565373] [<ffffffff81146b45>] ? ep_call_nested.constprop.5+0xd0/0xe1
[ 23.565381] [<ffffffff811467c6>] ? ep_poll_wakeup_proc+0x3c/0x3c
[ 23.565396] [<ffffffff81110f08>] ? drop_file_write_access+0x45/0x45
[ 23.565407] [<ffffffff81147673>] sys_epoll_ctl+0x17d/0x20b
[ 23.565417] [<ffffffff814ac3d2>] system_call_fastpath+0x16/0x1b
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 9026fc9..b5cf4a5 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -481,7 +481,8 @@ static int ep_scan_ready_list(struct eventpoll *ep,
* We need to lock this because we could be hit by
* eventpoll_release_file() and epoll_ctl().
*/
- mutex_lock(&ep->mtx);
+ if (!mutex_trylock(&ep->mtx))
+ return 0;
/*
* Steal the ready list, and re-init the original one to the
--
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