Patch "fs/dcache.c: fix spin lockup issue on nlru->lock" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

    fs/dcache.c: fix spin lockup issue on nlru->lock

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fs-dcache.c-fix-spin-lockup-issue-on-nlru-lock.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Sahitya Tummala <[email protected]>
Date: Mon, 10 Jul 2017 15:50:00 -0700
Subject: fs/dcache.c: fix spin lockup issue on nlru->lock

From: Sahitya Tummala <[email protected]>

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.

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

@@ -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);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mm-list_lru.c-fix-list_lru_count_node-to-be-race-free.patch queue-4.4/fs-dcache.c-fix-spin-lockup-issue-on-nlru-lock.patch