linux-kernel - [PATCH] mm: fs: fix lru_cache_disabled race in bh

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Message-Id: <20220308180709.2017638-1-minchan@kernel.org>
Date:   Tue,  8 Mar 2022 10:07:09 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Chris Goldsworthy <cgoldswo@...eaurora.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        John Dias <joaodias@...gle.com>,
        Minchan Kim <minchan@...nel.org>
Subject: [PATCH] mm: fs: fix lru_cache_disabled race in bh_lru

Check lru_cache_disabled under bh_lru_lock. Otherwise, it could
introduce race below and it fails to migrate pages containing
buffer_head.

   CPU 0					CPU 1

bh_lru_install
                                       lru_cache_disable
  lru_cache_disabled = false
                                       atomic_inc(&lru_disable_count);
				       invalidate_bh_lrus_cpu of CPU 0
				       bh_lru_lock
				       __invalidate_bh_lrus
				       bh_lru_unlock
  bh_lru_lock
  install the bh
  bh_lru_unlock

Fixes: 8cc621d2f45d (mm: fs: invalidate BH LRU during page migration)
Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 fs/buffer.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 8e112b6bd371..c76a8ef60a75 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1235,16 +1235,18 @@ static void bh_lru_install(struct buffer_head *bh)
 	int i;
 
 	check_irqs_on();
+	bh_lru_lock();
+
 	/*
 	 * the refcount of buffer_head in bh_lru prevents dropping the
 	 * attached page(i.e., try_to_free_buffers) so it could cause
 	 * failing page migration.
 	 * Skip putting upcoming bh into bh_lru until migration is done.
 	 */
-	if (lru_cache_disabled())
+	if (lru_cache_disabled()) {
+		bh_lru_unlock();
 		return;
-
-	bh_lru_lock();
+	}
 
 	b = this_cpu_ptr(&bh_lrus);
 	for (i = 0; i < BH_LRU_SIZE; i++) {
-- 
2.35.1.616.g0bdcbb4464-goog