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Subject[PATCH 4.4 171/268] btrfs: Fix possible softlock on single core machines
4.4-stable review patch.  If anyone has any objections, please let me know.


From: Nikolay Borisov <>

[ Upstream commit 1e1c50a929bc9e49bc3f9935b92450d9e69f8158 ]

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 <>
Reviewed-by: David Sterba <>
Signed-off-by: David Sterba <>
Signed-off-by: Sasha Levin <>
Signed-off-by: Greg Kroah-Hartman <>
fs/btrfs/extent-tree.c | 1 +
1 file changed, 1 insertion(+)

--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4392,6 +4392,7 @@ again:
if (wait_for_alloc) {
wait_for_alloc = 0;
+ cond_resched();
goto again;

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