fs/dcache.c: add cond_resched() in shrink_dentry_list()

As previously reported (https://patchwork.kernel.org/patch/8642031/)
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.

David said: I've found hundreds of occurrences of warnings that we emit
when need_resched stays set for a prolonged period of time with the
stack trace that is included in the change log.

Link: http://lkml.kernel.org/r/1521718946-31521-1-git-send-email-nborisov@suse.com
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/dcache.c b/fs/dcache.c
index 915816e..86d2de6 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1052,6 +1052,8 @@ static void shrink_dentry_list(struct list_head *list)
        while (!list_empty(list)) {
                struct dentry *dentry, *parent;
 
+               cond_resched();
+
                dentry = list_entry(list->prev, struct dentry, d_lru);
                spin_lock(&dentry->d_lock);
                rcu_read_lock();
@@ -1205,7 +1207,6 @@ 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);
@@ -1487,7 +1488,6 @@ void shrink_dcache_parent(struct dentry *parent)
                        break;
 
                shrink_dentry_list(&data.dispose);
-               cond_resched();
        }
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
@@ -1614,7 +1614,6 @@ void d_invalidate(struct dentry *dentry)
                        detach_mounts(data.mountpoint);
                        dput(data.mountpoint);
                }
-               cond_resched();
        }
 }
 EXPORT_SYMBOL(d_invalidate);