Both existing callers of __chunk_is_tracked() address very rare races
with conflicting I/O. In both cases the potential for conflict is so
improbable that the use of msleep(1) is warranted.
The justification for the pending_complete() call was introduced with
commit a8d41b59f3f5a7ac19452ef442a7fc1b5fa17366 ("dm snapshot: fix race
during exception creation").
In the case of snapshot_merge_process(), conflicting writes on a chunk
that is about to be merged is also quite rare. For example, a linear
write to a device that has a background merge active, e.g. the root
device, will only trigger the msleep(1) once.
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
---
drivers/md/dm-snap.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)
Index: linux-2.6/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-snap.c
+++ linux-2.6/drivers/md/dm-snap.c
@@ -236,6 +236,16 @@ static int __chunk_is_tracked(struct dm_
}
/*
+ * This conflicting I/O is extremely improbable in the caller,
+ * so msleep(1) is sufficient and there is no need for a wait queue.
+ */
+static void __check_for_conflicting_io(struct dm_snapshot *s, chunk_t chunk)
+{
+ while (__chunk_is_tracked(s, chunk))
+ msleep(1);
+}
+
+/*
* One of these per registered origin, held in the snapshot_origins hash
*/
struct origin {
@@ -832,10 +842,8 @@ test_again:
up_write(&s->lock);
/* Wait until writes to all 'linear_chunks' drain */
- for (i = 0; i < linear_chunks; i++) {
- while (__chunk_is_tracked(s, old_chunk + i))
- msleep(1);
- }
+ for (i = 0; i < linear_chunks; i++)
+ __check_for_conflicting_io(s, old_chunk + i);
dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, merge_callback, s);
return;
@@ -1340,12 +1348,8 @@ static void pending_complete(struct dm_s
goto out;
}
- /*
- * Check for conflicting reads. This is extremely improbable,
- * so msleep(1) is sufficient and there is no need for a wait queue.
- */
- while (__chunk_is_tracked(s, pe->e.old_chunk))
- msleep(1);
+ /* Check for conflicting reads */
+ __check_for_conflicting_io(s, pe->e.old_chunk);
/*
* Add a proper exception, and remove the
--
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