[PULL,01/15] mirror: Don't call job_pause_point() under graph lock

Commit Message

Kevin Wolf March 18, 2024, 1:01 p.m. UTC

Calling job_pause_point() while holding the graph reader lock
potentially results in a deadlock: bdrv_graph_wrlock() first drains
everything, including the mirror job, which pauses it. The job is only
unpaused at the end of the drain section, which is when the graph writer
lock has been successfully taken. However, if the job happens to be
paused at a pause point where it still holds the reader lock, the writer
lock can't be taken as long as the job is still paused.

Mark job_pause_point() as GRAPH_UNLOCKED and fix mirror accordingly.

Cc: [e-mail]
Buglink: https://issues.redhat.com/browse/RHEL-28125
Fixes: 004915a96a7a ("block: Protect bs->backing with graph_lock")
Signed-off-by: Kevin Wolf <[e-mail]>
Message-ID: <[e-mail]>
Reviewed-by: Eric Blake <[e-mail]>
Signed-off-by: Kevin Wolf <[e-mail]>
---
 include/qemu/job.h |  2 +-
 block/mirror.c     | 10 ++++++----
 2 files changed, 7 insertions(+), 5 deletions(-)

Patch

diff --git a/include/qemu/job.h b/include/qemu/job.h
index 9ea98b5927..2b873f2576 100644
--- a/include/qemu/job.h
+++ b/include/qemu/job.h
@@ -483,7 +483,7 @@  void job_enter(Job *job);
  *
  * Called with job_mutex *not* held.
  */
-void coroutine_fn job_pause_point(Job *job);
+void coroutine_fn GRAPH_UNLOCKED job_pause_point(Job *job);
 
 /**
  * @job: The job that calls the function.
diff --git a/block/mirror.c b/block/mirror.c
index 5145eb53e1..1bdce3b657 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -479,9 +479,9 @@  static unsigned mirror_perform(MirrorBlockJob *s, int64_t offset,
     return bytes_handled;
 }
 
-static void coroutine_fn GRAPH_RDLOCK mirror_iteration(MirrorBlockJob *s)
+static void coroutine_fn GRAPH_UNLOCKED mirror_iteration(MirrorBlockJob *s)
 {
-    BlockDriverState *source = s->mirror_top_bs->backing->bs;
+    BlockDriverState *source;
     MirrorOp *pseudo_op;
     int64_t offset;
     /* At least the first dirty chunk is mirrored in one iteration. */
@@ -489,6 +489,10 @@  static void coroutine_fn GRAPH_RDLOCK mirror_iteration(MirrorBlockJob *s)
     bool write_zeroes_ok = bdrv_can_write_zeroes_with_unmap(blk_bs(s->target));
     int max_io_bytes = MAX(s->buf_size / MAX_IN_FLIGHT, MAX_IO_BYTES);
 
+    bdrv_graph_co_rdlock();
+    source = s->mirror_top_bs->backing->bs;
+    bdrv_graph_co_rdunlock();
+
     bdrv_dirty_bitmap_lock(s->dirty_bitmap);
     offset = bdrv_dirty_iter_next(s->dbi);
     if (offset < 0) {
@@ -1066,9 +1070,7 @@  static int coroutine_fn mirror_run(Job *job, Error **errp)
                 mirror_wait_for_free_in_flight_slot(s);
                 continue;
             } else if (cnt != 0) {
-                bdrv_graph_co_rdlock();
                 mirror_iteration(s);
-                bdrv_graph_co_rdunlock();
             }
         }