qemu-kvm/SOURCES/kvm-blockjob-Lie-better-in-child_job_drained_poll.patch

From 7c16384f8ce4d46d6baa11db376c488ed8478744 Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Wed, 10 Oct 2018 20:22:06 +0100
Subject: [PATCH 40/49] blockjob: Lie better in child_job_drained_poll()

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20181010202213.7372-28-kwolf@redhat.com>
Patchwork-id: 82616
O-Subject: [RHEL-8 qemu-kvm PATCH 37/44] blockjob: Lie better in child_job_drained_poll()
Bugzilla: 1637976
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: John Snow <jsnow@redhat.com>
RH-Acked-by: Thomas Huth <thuth@redhat.com>

Block jobs claim in .drained_poll() that they are in a quiescent state
as soon as job->deferred_to_main_loop is true. This is obviously wrong,
they still have a completion BH to run. We only get away with this
because commit 91af091f923 added an unconditional aio_poll(false) to the
drain functions, but this is bypassing the regular drain mechanisms.

However, just removing this and telling that the job is still active
doesn't work either: The completion callbacks themselves call drain
functions (directly, or indirectly with bdrv_reopen), so they would
deadlock then.

As a better lie, tell that the job is active as long as the BH is
pending, but falsely call it quiescent from the point in the BH when the
completion callback is called. At this point, nested drain calls won't
deadlock because they ignore the job, and outer drains will wait for the
job to really reach a quiescent state because the callback is already
running.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit b5a7a0573530698ee448b063ac01d485e30446bd)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 blockjob.c         |  2 +-
 include/qemu/job.h |  3 +++
 job.c              | 11 ++++++++++-
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/blockjob.c b/blockjob.c
index 8d27e8e..617d86f 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -164,7 +164,7 @@ static bool child_job_drained_poll(BdrvChild *c)
     /* An inactive or completed job doesn't have any pending requests. Jobs
      * with !job->busy are either already paused or have a pause point after
      * being reentered, so no job driver code will run before they pause. */
-    if (!job->busy || job_is_completed(job) || job->deferred_to_main_loop) {
+    if (!job->busy || job_is_completed(job)) {
         return false;
     }
 
diff --git a/include/qemu/job.h b/include/qemu/job.h
index 35ac7a9..d1710f3 100644
--- a/include/qemu/job.h
+++ b/include/qemu/job.h
@@ -76,6 +76,9 @@ typedef struct Job {
      * Set to false by the job while the coroutine has yielded and may be
      * re-entered by job_enter(). There may still be I/O or event loop activity
      * pending. Accessed under block_job_mutex (in blockjob.c).
+     *
+     * When the job is deferred to the main loop, busy is true as long as the
+     * bottom half is still pending.
      */
     bool busy;
 
diff --git a/job.c b/job.c
index 47b5a11..42af9e2 100644
--- a/job.c
+++ b/job.c
@@ -852,7 +852,16 @@ static void job_exit(void *opaque)
     AioContext *ctx = job->aio_context;
 
     aio_context_acquire(ctx);
+
+    /* This is a lie, we're not quiescent, but still doing the completion
+     * callbacks. However, completion callbacks tend to involve operations that
+     * drain block nodes, and if .drained_poll still returned true, we would
+     * deadlock. */
+    job->busy = false;
+    job_event_idle(job);
+
     job_completed(job);
+
     aio_context_release(ctx);
 }
 
@@ -867,8 +876,8 @@ static void coroutine_fn job_co_entry(void *opaque)
     assert(job && job->driver && job->driver->run);
     job_pause_point(job);
     job->ret = job->driver->run(job, &job->err);
-    job_event_idle(job);
     job->deferred_to_main_loop = true;
+    job->busy = true;
     aio_bh_schedule_oneshot(qemu_get_aio_context(), job_exit, job);
 }
 
-- 
1.8.3.1