Geng Jichao reported a rather complex deadlock involving several
moving parts:

1) readahead is issued against an inode and some of its pages are locked
   while the read is in flight

2) the same inode is evicted from the cache, and this task gets stuck
   waiting for the page lock because of the above readahead

3) another task is processing a reply trace, and looks up the inode
   being evicted while holding the s_mutex. That ends up waiting for the
   eviction to complete

4) a write reply for an unrelated inode is then processed in the
   ceph_con_workfn job. It calls ceph_check_caps after putting wrbuffer
   caps, and that gets stuck waiting on the s_mutex held by 3.

The reply to "1" is stuck behind the write reply in "4", so we deadlock
at that point.

This patch changes the trace processing to call ceph_get_inode outside
of the s_mutex and snap_rwsem, which should break the cycle above.

[ idryomov: break unnecessarily long lines ]

URL: https://tracker.ceph.com/issues/47998Reported-by: Geng Jichao <gengjichao@jd.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>