linux-kernel - [PATCH 3.16 107/366] btrfs: Fix possible softlock on single core machines

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Message-ID: <lsq.1539530741.472808259@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David Sterba" <dsterba@...e.com>,
        "Nikolay Borisov" <nborisov@...e.com>
Subject: [PATCH 3.16 107/366] btrfs: Fix possible softlock on single core
 machines

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Nikolay Borisov <nborisov@...e.com>

commit 1e1c50a929bc9e49bc3f9935b92450d9e69f8158 upstream.

do_chunk_alloc implements a loop checking whether there is a pending
chunk allocation and if so causes the caller do loop. Generally this
loop is executed only once, however testing with btrfs/072 on a single
core vm machines uncovered an extreme case where the system could loop
indefinitely. This is due to a missing cond_resched when loop which
doesn't give a chance to the previous chunk allocator finish its job.

The fix is to simply add the missing cond_resched.

Fixes: 6d74119f1a3e ("Btrfs: avoid taking the chunk_mutex in do_chunk_alloc")
Signed-off-by: Nikolay Borisov <nborisov@...e.com>
Reviewed-by: David Sterba <dsterba@...e.com>
Signed-off-by: David Sterba <dsterba@...e.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/btrfs/extent-tree.c | 1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3969,6 +3969,7 @@ again:
 	if (wait_for_alloc) {
 		mutex_unlock(&fs_info->chunk_mutex);
 		wait_for_alloc = 0;
+		cond_resched();
 		goto again;
 	}