[Devel] [PATCH RHEL7 COMMIT] ms/fs/dcache.c: fix spin lockup issue on nlru->lock

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-957.21.3.vz7.106.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-957.21.3.vz7.106.3
------>
commit b9edbc15cf20fbfa8b953daf738a6b55b84f68b8
Author: Sahitya Tummala <stummala at codeaurora.org>
Date:   Mon Jul 1 12:37:17 2019 +0300

    ms/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:
    
      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 at codeaurora.org>
    Suggested-by: Jan Kara <jack at suse.cz>
    Suggested-by: Vladimir Davydov <vdavydov.dev at gmail.com>
    Acked-by: Vladimir Davydov <vdavydov.dev at gmail.com>
    Cc: Alexander Polakov <apolyakov at beget.ru>
    Cc: Al Viro <viro at zeniv.linux.org.uk>
    Cc: <stable at vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    (cherry picked from commit b17c070fb624cf10162cf92ea5e1ec25cd8ac176)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/dcache.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 175ae8c7acb8..09a11fbd94ea 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1056,11 +1056,12 @@ void shrink_dcache_sb(struct super_block *sb)
 		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);