linux-ext4 - [PATCH] jbd2: flush_descriptor(): Do not decrease buffer head's ref count

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Message-Id: <20190725131409.32172-1-chandan@linux.ibm.com>
Date:   Thu, 25 Jul 2019 18:44:09 +0530
From:   Chandan Rajendra <chandan@...ux.ibm.com>
To:     linux-ext4@...r.kernel.org
Cc:     Chandan Rajendra <chandan@...ux.ibm.com>, tytso@....edu,
        adilger.kernel@...ger.ca
Subject: [PATCH] jbd2: flush_descriptor(): Do not decrease buffer head's ref count

When executing generic/388 on a ppc64le machine, we notice the following
call trace,

VFS: brelse: Trying to free free buffer
WARNING: CPU: 0 PID: 6637 at /root/repos/linux/fs/buffer.c:1195 __brelse+0x84/0xc0

Call Trace:
 __brelse+0x80/0xc0 (unreliable)
 invalidate_bh_lru+0x78/0xc0
 on_each_cpu_mask+0xa8/0x130
 on_each_cpu_cond_mask+0x130/0x170
 invalidate_bh_lrus+0x44/0x60
 invalidate_bdev+0x38/0x70
 ext4_put_super+0x294/0x560
 generic_shutdown_super+0xb0/0x170
 kill_block_super+0x38/0xb0
 deactivate_locked_super+0xa4/0xf0
 cleanup_mnt+0x164/0x1d0
 task_work_run+0x110/0x160
 do_notify_resume+0x414/0x460
 ret_from_except_lite+0x70/0x74

The above occurs due to the following sequence of events,
1. Get a buffer head for holding a descriptor buffer for a Revoke
   record. This causes buffer_head->b_count to have a value of 2,
   - The first ref count corresponds to JBD code holding a reference to the
     buffer head.
   - Another ref count correponds to adding the buffer head to the
     per-cpu LRU list.
   The buffer head is also added to the list of log_bufs tracking the
   buffer heads that need to be written to the on-disk journal.
2. When writing the revoke record to the disk, if journal is aborted,
   flush_descriptor() gives up one ref count by invoking put_bh().
3. jbd2_journal_commit_transaction() then loops across the buffer heads
   in the log_bufs list. While doing so, it decrements the buffer head's ref
   count to 0 by invoking __brelse().
4. On unmount, invalidate_bdev() invokes __brelse() via
   invalidate_bh_lru() to remove the buffer head from the per-cpu LRU
   list. Here __brelse() prints the call trace shown above since
   buffer_head->b_count already has a value of 0.

Hence this commit removes the call to put_bh() inside
flush_descriptor().

Signed-off-by: Chandan Rajendra <chandan@...ux.ibm.com>
---
 fs/jbd2/revoke.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/jbd2/revoke.c b/fs/jbd2/revoke.c
index 69b9bc329964..f08073d7bbf5 100644
--- a/fs/jbd2/revoke.c
+++ b/fs/jbd2/revoke.c
@@ -638,10 +638,8 @@ static void flush_descriptor(journal_t *journal,
 {
 	jbd2_journal_revoke_header_t *header;
 
-	if (is_journal_aborted(journal)) {
-		put_bh(descriptor);
+	if (is_journal_aborted(journal))
 		return;
-	}
 
 	header = (jbd2_journal_revoke_header_t *)descriptor->b_data;
 	header->r_count = cpu_to_be32(offset);
-- 
2.19.1