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Re: [PATCH 2/2] mirror: Wait only for in-flight operations

From: Eric Blake
Subject: Re: [PATCH 2/2] mirror: Wait only for in-flight operations
Date: Wed, 25 Mar 2020 12:39:01 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 3/25/20 12:23 PM, Kevin Wolf wrote:
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 <address@hidden>
  block/mirror.c | 8 +++++++-
  1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index 393131b135..7fef52ded2 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);

Looks like a one-way transition - op->is_in_flight always starts as false, and only ever gets set to true (once the op is finished, op is no longer needed). And being more selective on what you wait for here does look like it should work in more cases than what patch 1 reverted.

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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