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[Qemu-block] [PATCH for-2.9] thread-pool: add missing qemu_bh_cancel in

From: Peter Lieven
Subject: [Qemu-block] [PATCH for-2.9] thread-pool: add missing qemu_bh_cancel in completion function
Date: Thu, 16 Mar 2017 17:02:49 +0100

commit 3c80ca15 fixed a deadlock scenarion with nested aio_poll invocations.

However, the rescheduling of the completion BH introcuded unnecessary spinning
in the main-loop. On very fast file backends this can even lead to the
"WARNING: I/O thread spun for 1000 iterations" message popping up.

Callgrind reports about 3-4% less instructions with this patch running
qemu-img bench on a ramdisk based VMDK file.

Fixes: 3c80ca158c96ff902a30883a8933e755988948b1
Cc: address@hidden
Signed-off-by: Peter Lieven <address@hidden>
 util/thread-pool.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/util/thread-pool.c b/util/thread-pool.c
index ce6cd30..610646d 100644
--- a/util/thread-pool.c
+++ b/util/thread-pool.c
@@ -188,6 +188,13 @@ restart:
             elem->common.cb(elem->common.opaque, elem->ret);
+            /* We can safely cancel the completion_bh here regardless of 
+             * else having scheduled it meanwhile because we reenter the
+             * completion function anyway (goto restart).
+             */
+            qemu_bh_cancel(pool->completion_bh);
             goto restart;
         } else {

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