[PATCH 4.4 26/57] fs/dcache.c: fix spin lockup issue on nlru->lock

Greg Kroah-Hartman
2017-07-19T11:35:36Z

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/dcache.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1128,11 +1128,12 @@ void shrink_dcache_sb(struct super_block
LIST_HEAD(dispose);

freed = list_lru_walk(&sb->s_dentry_lru,
- dentry_lru_isolate_shrink, &dispose, UINT_MAX);
+ dentry_lru_isolate_shrink, &dispose, 1024);

this_cpu_sub(nr_dentry_unused, freed);
shrink_dentry_list(&dispose);
- } while (freed > 0);
+ cond_resched();
+ } while (list_lru_count(&sb->s_dentry_lru) > 0);
}
EXPORT_SYMBOL(shrink_dcache_sb);