[PATCH v2] dcache: Add cond_resched in shrink_dentry_list

[Nikolay Borisov <nborisov@suse.com>]

As previously [1] reported it's possible to call shrink_dentry_list
with a large number of dentries (> 10000). This, in turn, could
trigger the softlockup detector and possibly trigger a panic.
In addition to the unmount path being vulnerable to this scenario,
at SuSE we've observed similar situation happening during process
exit on processes that touch a lot of dentries. Here is an excerpt
from a crash dump. The number after the colon are the number of
dentries on the list passed to shrink_dentry_list:

PID 99760: 10722
PID 107530: 215
PID 108809: 24134
PID 108877: 21331
PID 141708: 16487

So we want to kill between 15k-25k dentries without yielding.

And one possible call stack looks like:

4 [ffff8839ece41db0] _raw_spin_lock at ffffffff8152a5f8
5 [ffff8839ece41db0] evict at ffffffff811c3026
6 [ffff8839ece41dd0] __dentry_kill at ffffffff811bf258
7 [ffff8839ece41df0] shrink_dentry_list at ffffffff811bf593
8 [ffff8839ece41e18] shrink_dcache_parent at ffffffff811bf830
9 [ffff8839ece41e50] proc_flush_task at ffffffff8120dd61
10 [ffff8839ece41ec0] release_task at ffffffff81059ebd
11 [ffff8839ece41f08] do_exit at ffffffff8105b8ce
12 [ffff8839ece41f78] sys_exit at ffffffff8105bd53
13 [ffff8839ece41f80] system_call_fastpath at ffffffff81532909

While some of the callers of shrink_dentry_list do use cond_resched,
this is not sufficient to prevent softlockups. So just move
cond_resched into shrink_dentry_list from its callers.

[1] https://patchwork.kernel.org/patch/8642031/

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---

V2: 
 * Fix typo in conD_resched
 * Actually compile test it 
 fs/dcache.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 8945e6cabd93..d9f3a53b5898 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -982,6 +982,9 @@ static void shrink_dentry_list(struct list_head *list)
 
 	while (!list_empty(list)) {
 		struct inode *inode;
+
+		cond_resched();
+
 		dentry = list_entry(list->prev, struct dentry, d_lru);
 		spin_lock(&dentry->d_lock);
 		parent = lock_parent(dentry);
@@ -1177,7 +1180,6 @@ void shrink_dcache_sb(struct super_block *sb)
 
 		this_cpu_sub(nr_dentry_unused, freed);
 		shrink_dentry_list(&dispose);
-		cond_resched();
 	} while (list_lru_count(&sb->s_dentry_lru) > 0);
 }
 EXPORT_SYMBOL(shrink_dcache_sb);
@@ -1459,7 +1461,6 @@ void shrink_dcache_parent(struct dentry *parent)
 			break;
 
 		shrink_dentry_list(&data.dispose);
-		cond_resched();
 	}
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
@@ -1586,7 +1587,6 @@ void d_invalidate(struct dentry *dentry)
 			detach_mounts(data.mountpoint);
 			dput(data.mountpoint);
 		}
-		cond_resched();
 	}
 }
 EXPORT_SYMBOL(d_invalidate);
-- 
2.7.4