From ba6d773a79eef6f59687771bc5664814d96b6b03 Mon Sep 17 00:00:00 2001
From: Sergio Lopez Pascual <slp@redhat.com>
Date: Mon, 9 Sep 2019 12:46:40 +0200
Subject: [PATCH 4/4] mirror: Confirm we're quiesced only if the job is paused
 or cancelled

RH-Author: Sergio Lopez Pascual <slp@redhat.com>
Message-id: <20190909124640.53625-2-slp@redhat.com>
Patchwork-id: 90339
O-Subject: [RHEL-7.7.z qemu-kvm-rhev PATCH 1/1] mirror: Confirm we're quiesced only if the job is paused or cancelled
Bugzilla: 1665256
RH-Acked-by: Kevin Wolf <kwolf@redhat.com>
RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
RH-Acked-by: John Snow <jsnow@redhat.com>

While child_job_drained_begin() calls to job_pause(), the job doesn't
actually transition between states until it runs again and reaches a
pause point. This means bdrv_drained_begin() may return with some jobs
using the node still having 'busy == true'.

As a consequence, block_job_detach_aio_context() may get into a
deadlock, waiting for the job to be actually paused, while the coroutine
servicing the job is yielding and doesn't get the opportunity to get
scheduled again. This situation can be reproduced by issuing a
'block-commit' immediately followed by a 'device_del'.

To ensure bdrv_drained_begin() only returns when the jobs have been
paused, we change mirror_drained_poll() to only confirm it's quiesced
when job->paused == true and there aren't any in-flight requests, except
if we reached that point by a drained section initiated by the
mirror/commit job itself.

The other block jobs shouldn't need any changes, as the default
drained_poll() behavior is to only confirm it's quiesced if the job is
not busy or completed.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 5e771752a1ffba3a99d7d75b6d492b4a86b59e1b)
Signed-off-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 block/mirror.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 55dc94f..48c907f 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -72,6 +72,7 @@ typedef struct MirrorBlockJob {
     int max_iov;
     bool initial_zeroing_ongoing;
     bool prepared;
+    bool in_drain;
 } MirrorBlockJob;
 
 typedef struct MirrorOp {
@@ -551,6 +552,7 @@ static int mirror_exit_common(Job *job)
 
         /* The mirror job has no requests in flight any more, but we need to
          * drain potential other users of the BDS before changing the graph. */
+        assert(s->in_drain);
         bdrv_drained_begin(target_bs);
         bdrv_replace_node(to_replace, target_bs, &local_err);
         bdrv_drained_end(target_bs);
@@ -587,6 +589,7 @@ static int mirror_exit_common(Job *job)
     blk_insert_bs(bjob->blk, mirror_top_bs, &error_abort);
 
     bdrv_drained_end(src);
+    s->in_drain = false;
     bdrv_unref(mirror_top_bs);
     bdrv_unref(src);
 
@@ -860,10 +863,12 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
              */
             trace_mirror_before_drain(s, cnt);
 
+            s->in_drain = true;
             bdrv_drained_begin(bs);
             cnt = bdrv_get_dirty_count(s->dirty_bitmap);
             if (cnt > 0 || mirror_flush(s) < 0) {
                 bdrv_drained_end(bs);
+                s->in_drain = false;
                 continue;
             }
 
@@ -911,6 +916,7 @@ immediate_exit:
     bdrv_dirty_iter_free(s->dbi);
 
     if (need_drain) {
+        s->in_drain = true;
         bdrv_drained_begin(bs);
     }
 
@@ -979,6 +985,16 @@ static void mirror_pause(Job *job)
 static bool mirror_drained_poll(BlockJob *job)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common);
+
+    /* If the job isn't paused nor cancelled, we can't be sure that it won't
+     * issue more requests. We make an exception if we've reached this point
+     * from one of our own drain sections, to avoid a deadlock waiting for
+     * ourselves.
+     */
+    if (!s->common.job.paused && !s->common.job.cancelled && !s->in_drain) {
+        return true;
+    }
+
     return !!s->in_flight;
 }
 
-- 
1.8.3.1