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

__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 -

[<ffffff8eca0fb0bc>] spin_bug+0x90
[<ffffff8eca0fb220>] do_raw_spin_lock+0xfc
[<ffffff8ecafb7798>] _raw_spin_lock+0x28
[<ffffff8eca1ae884>] list_lru_add+0x28
[<ffffff8eca1f5dac>] dput+0x1c8
[<ffffff8eca1eb46c>] path_put+0x20
[<ffffff8eca1eb73c>] terminate_walk+0x3c
[<ffffff8eca1eee58>] path_lookupat+0x100
[<ffffff8eca1f00fc>] filename_lookup+0x6c
[<ffffff8eca1f0264>] user_path_at_empty+0x54
[<ffffff8eca1e066c>] SyS_faccessat+0xd0
[<ffffff8eca084e30>] el0_svc_naked+0x24

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

[<ffffff8eca1f5fd0>] d_lru_shrink_move+0x34
[<ffffff8eca1f6180>] dentry_lru_isolate_shrink+0x48
[<ffffff8eca1aeafc>] __list_lru_walk_one.isra.10+0x94
[<ffffff8eca1aec34>] list_lru_walk_node+0x40
[<ffffff8eca1f6620>] shrink_dcache_sb+0x60
[<ffffff8eca1e56a8>] do_remount_sb+0xbc
[<ffffff8eca1e583c>] do_emergency_remount+0xb0
[<ffffff8eca0ba510>] process_one_work+0x228
[<ffffff8eca0bb158>] worker_thread+0x2e0
[<ffffff8eca0c040c>] kthread+0xf4
[<ffffff8eca084dd0>] 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.

Change-Id: I6df8d74cbc4c5ceac294f2d0784702cf096e84be
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>
Cc: Alexander Polakov <apolyakov@beget.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Patch-mainline: linux-mm @ 29/06/17, 09:09:35
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>