dm kcopyd: fix callback race

If the 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, the kcopyd callback could be invoked from the
thread that called dm_kcopyd_copy instead of the kcopyd workqueue.

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

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 mean that exception store callbacks are called twice
for the same exception - a likely reason for crashes reported inside
pending_complete() / remove_exception().

This patch fixes two problems:

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

- Fix: hand over the completion callback to the kcopyd thread.

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.

- Fix: pass job->write_err to the callback to report all error bits
  (it is done already in run_complete_job)

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-kcopyd.c

@@ -511,13 +511,16 @@ static void segment_complete(int read_err, unsigned long write_err,
 	} else if (atomic_dec_and_test(&job->sub_jobs)) {
 
 		/*
-		 * To avoid a race we must keep the job around
+		 * Queue the completion callback to the kcopyd thread.
-		 * until after the notify function has completed.
+		 *
-		 * Otherwise the client may try and stop the job
+		 * Some callers assume that all the completions are called
-		 * after we've completed.
+		 * from a single thread and don't race with each other.
+		 *
+		 * We must not call the callback directly here because this
+		 * code may not be executing in the thread.
 		 */
-		job->fn(read_err, write_err, job->context);
+		push(&kc->complete_jobs, job);
-		mempool_free(job, job->kc->job_pool);
+		wake(kc);
 	}
 }
 
@@ -530,6 +533,8 @@ static void split_job(struct kcopyd_job *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);