epoll: fix spurious lockdep warnings
authorNelson Elhage <nelhage@nelhage.com>
Tue, 1 Nov 2011 00:13:14 +0000 (17:13 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 1 Nov 2011 00:30:57 +0000 (17:30 -0700)
commitd8805e633e054c816c47cb6e727c81f156d9253d
tree8f7151e635a38593d68eae40586449627d835965
parent15662b3e8644905032c2e26808401a487d4e90c1
epoll: fix spurious lockdep warnings

epoll can acquire recursively acquire ep->mtx on multiple "struct
eventpoll"s at once in the case where one epoll fd is monitoring another
epoll fd.  This is perfectly OK, since we're careful about the lock
ordering, but it causes spurious lockdep warnings.  Annotate the recursion
using mutex_lock_nested, and add a comment explaining the nesting rules
for good measure.

Recent versions of systemd are triggering this, and it can also be
demonstrated with the following trivial test program:

--------------------8<--------------------

int main(void) {
   int e1, e2;
   struct epoll_event evt = {
       .events = EPOLLIN
   };

   e1 = epoll_create1(0);
   e2 = epoll_create1(0);
   epoll_ctl(e1, EPOLL_CTL_ADD, e2, &evt);
   return 0;
}
--------------------8<--------------------

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Tested-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Nelson Elhage <nelhage@nelhage.com>
Acked-by: Jason Baron <jbaron@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: <stable@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
fs/eventpoll.c