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[dm-devel] [PATCH 3/3] kcopyd+snapshots race condition



Under specific circumstances, kcopyd callback could be called from
the thread that called dm_kcopyd_copy not from kcopyd workqueue.

dm_kcopyd_copy -> split_job -> segment_complete -> job->fn()

This code path is taken if thread calling dm_kcopyd_copy is delayed due to
scheduling inside split_job/segment_complete and the subjobs complete before
the loop in split_job completes.

Snapshots depend on the fact that callbacks are called from the singlethreaded
kcopyd workqueue and expect that there is no racing between individual
callbacks. The racing between callbacks can lead to corruption of exception
store and it can also cause that exception store callbacks are called twice
for the same exception --- a likely reason for crashes inside
pending_complete() / remove_exception().


When I reviewed kcopyd further, I found four total problems:

1. job->fn being called from the thread that submitted the job (see above).

2. job->fn(read_err, write_err, job->context); in segment_complete
reports the error of the last subjob, not the union of all errors.

3. This comment is bogus ---- the jobs are not cancelable. It is leftover from
   some time when developers thought about adding cancel possibility.
/*
 * To avoid a race we must keep the job around
 * until after the notify function has completed.
 * Otherwise the client may try and stop the job
 * after we've completed.
 */
job->fn(read_err, write_err, job->context);
mempool_free(job, job->kc->job_pool);

4. Master jobs and subjobs are allocated from the same mempool.
Completion and freeing of a master job depends on successful allocation of
all subjobs -> deadlock.


This patch moves completion of master jobs to run_complete_job (being called
from the kcopyd workqueue). The patch fixes problems 1, 2, 3.

Problem 4 will need more changes and another patch.

Signed-off-by: Mikulas Patocka <mpatocka redhat com>

---
 drivers/md/dm-kcopyd.c |   12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

Index: linux-2.6.29-rc8-devel/drivers/md/dm-kcopyd.c
===================================================================
--- linux-2.6.29-rc8-devel.orig/drivers/md/dm-kcopyd.c	2009-03-18 20:39:19.000000000 +0100
+++ linux-2.6.29-rc8-devel/drivers/md/dm-kcopyd.c	2009-03-18 20:40:26.000000000 +0100
@@ -510,14 +510,8 @@ static void segment_complete(int read_er
 
 	} else if (atomic_dec_and_test(&job->sub_jobs)) {
 
-		/*
-		 * To avoid a race we must keep the job around
-		 * until after the notify function has completed.
-		 * Otherwise the client may try and stop the job
-		 * after we've completed.
-		 */
-		job->fn(read_err, write_err, job->context);
-		mempool_free(job, job->kc->job_pool);
+		push(&kc->complete_jobs, job);
+		wake(kc);
 	}
 }
 
@@ -530,6 +524,8 @@ static void split_job(struct kcopyd_job 
 {
 	int i;
 
+	atomic_inc(&job->kc->nr_jobs);
+
 	atomic_set(&job->sub_jobs, SPLIT_COUNT);
 	for (i = 0; i < SPLIT_COUNT; i++)
 		segment_complete(0, 0u, job);


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