[PATCH 3.2 40/74] fs/dcache.c: fix spin lockup issue on nlru->lock

[Ben Hutchings <ben@decadent.org.uk>]

3.2.94-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Sahitya Tummala <stummala@codeaurora.org>

commit b17c070fb624cf10162cf92ea5e1ec25cd8ac176 upstream.

__list_lru_walk_one() acquires nlru spin lock (nlru->lock) for longer
duration if there are more number of items in the lru list.  As per the
current code, it can hold the spin lock for upto maximum UINT_MAX
entries at a time.  So if there are more number of items in the lru
list, then "BUG: spinlock lockup suspected" is observed in the below
path:

  spin_bug+0x90
  do_raw_spin_lock+0xfc
  _raw_spin_lock+0x28
  list_lru_add+0x28
  dput+0x1c8
  path_put+0x20
  terminate_walk+0x3c
  path_lookupat+0x100
  filename_lookup+0x6c
  user_path_at_empty+0x54
  SyS_faccessat+0xd0
  el0_svc_naked+0x24

This nlru->lock is acquired by another CPU in this path -

  d_lru_shrink_move+0x34
  dentry_lru_isolate_shrink+0x48
  __list_lru_walk_one.isra.10+0x94
  list_lru_walk_node+0x40
  shrink_dcache_sb+0x60
  do_remount_sb+0xbc
  do_emergency_remount+0xb0
  process_one_work+0x228
  worker_thread+0x2e0
  kthread+0xf4
  ret_from_fork+0x10

Fix this lockup by reducing the number of entries to be shrinked from
the lru list to 1024 at once.  Also, add cond_resched() before
processing the lru list again.

Link: http://marc.info/?t=149722864900001&r=1&w=2
Link: http://lkml.kernel.org/r/1498707575-2472-1-git-send-email-stummala@codeaurora.org
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Suggested-by: Jan Kara <jack@suse.cz>
Suggested-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Alexander Polakov <apolyakov@beget.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: we don't hold the spin-lock for long, but adding
 cond_resched() looks like a good idea]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 fs/dcache.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -836,6 +836,7 @@ void shrink_dcache_sb(struct super_block
 		list_splice_init(&sb->s_dentry_lru, &tmp);
 		spin_unlock(&dcache_lru_lock);
 		shrink_dentry_list(&tmp);
+		cond_resched();
 		spin_lock(&dcache_lru_lock);
 	}
 	spin_unlock(&dcache_lru_lock);