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From: Kevin Wolf <kwolf@redhat.com>
To: qemu-block@nongnu.org
Cc: kwolf@redhat.com, jsnow@redhat.com, qemu-devel@nongnu.org,
	mreitz@redhat.com
Subject: [PATCH v2 2/2] mirror: Wait only for in-flight operations
Date: Thu, 26 Mar 2020 16:36:28 +0100
Message-ID: <20200326153628.4869-3-kwolf@redhat.com> (raw)
In-Reply-To: <20200326153628.4869-1-kwolf@redhat.com>

mirror_wait_for_free_in_flight_slot() just picks a random operation to
wait for. However, a MirrorOp is already in s->ops_in_flight when
mirror_co_read() waits for free slots, so if not enough slots are
immediately available, an operation can end up waiting for itself, or
two or more operations can wait for each other to complete, which
results in a hang.

Fix this by adding a flag to MirrorOp that tells us if the request is
already in flight (and therefore occupies slots that it will later
free), and picking only such operations for waiting.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1794692
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/mirror.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index 393131b135..88414d1653 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -102,6 +102,7 @@ struct MirrorOp {
 
     bool is_pseudo_op;
     bool is_active_write;
+    bool is_in_flight;
     CoQueue waiting_requests;
     Coroutine *co;
 
@@ -293,7 +294,9 @@ mirror_wait_for_any_operation(MirrorBlockJob *s, bool active)
          * caller of this function.  Since there is only one pseudo op
          * at any given time, we will always find some real operation
          * to wait on. */
-        if (!op->is_pseudo_op && op->is_active_write == active) {
+        if (!op->is_pseudo_op && op->is_in_flight &&
+            op->is_active_write == active)
+        {
             qemu_co_queue_wait(&op->waiting_requests, NULL);
             return;
         }
@@ -367,6 +370,7 @@ static void coroutine_fn mirror_co_read(void *opaque)
     /* Copy the dirty cluster.  */
     s->in_flight++;
     s->bytes_in_flight += op->bytes;
+    op->is_in_flight = true;
     trace_mirror_one_iteration(s, op->offset, op->bytes);
 
     ret = bdrv_co_preadv(s->mirror_top_bs->backing, op->offset, op->bytes,
@@ -382,6 +386,7 @@ static void coroutine_fn mirror_co_zero(void *opaque)
     op->s->in_flight++;
     op->s->bytes_in_flight += op->bytes;
     *op->bytes_handled = op->bytes;
+    op->is_in_flight = true;
 
     ret = blk_co_pwrite_zeroes(op->s->target, op->offset, op->bytes,
                                op->s->unmap ? BDRV_REQ_MAY_UNMAP : 0);
@@ -396,6 +401,7 @@ static void coroutine_fn mirror_co_discard(void *opaque)
     op->s->in_flight++;
     op->s->bytes_in_flight += op->bytes;
     *op->bytes_handled = op->bytes;
+    op->is_in_flight = true;
 
     ret = blk_co_pdiscard(op->s->target, op->offset, op->bytes);
     mirror_write_complete(op, ret);
@@ -1318,6 +1324,7 @@ static MirrorOp *coroutine_fn active_write_prepare(MirrorBlockJob *s,
         .offset             = offset,
         .bytes              = bytes,
         .is_active_write    = true,
+        .is_in_flight       = true,
     };
     qemu_co_queue_init(&op->waiting_requests);
     QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next);
-- 
2.20.1



  parent reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26 15:36 [PATCH v2 0/2] mirror: Fix hang (operation waiting for itself/circular dependency) Kevin Wolf
2020-03-26 15:36 ` [PATCH v2 1/2] Revert "mirror: Don't let an operation wait for itself" Kevin Wolf
2020-03-26 15:36 ` Kevin Wolf [this message]
2020-03-26 18:27   ` [PATCH v2 2/2] mirror: Wait only for in-flight operations Max Reitz
2020-03-26 15:53 ` [PATCH v2 0/2] mirror: Fix hang (operation waiting for itself/circular dependency) Eric Blake

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