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From 27fd652033779a16561160bef8aeda7f8f9c04be Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Wed, 10 Oct 2018 20:22:05 +0100
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Subject: [PATCH 39/49] block-backend: Decrease in_flight only after callback
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20181010202213.7372-27-kwolf@redhat.com>
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Patchwork-id: 82617
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O-Subject: [RHEL-8 qemu-kvm PATCH 36/44] block-backend: Decrease in_flight only after callback
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Bugzilla: 1637976
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: John Snow <jsnow@redhat.com>
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RH-Acked-by: Thomas Huth <thuth@redhat.com>
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Request callbacks can do pretty much anything, including operations that
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will yield from the coroutine (such as draining the backend). In that
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case, a decreased in_flight would be visible to other code and could
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lead to a drain completing while the callback hasn't actually completed
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yet.
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Note that reordering these operations forbids calling drain directly
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inside an AIO callback. As Paolo explains, indirectly calling it is
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okay:
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- Calling it through a coroutine is okay, because then
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bdrv_drained_begin() goes through bdrv_co_yield_to_drain() and you
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have in_flight=2 when bdrv_co_yield_to_drain() yields, then soon
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in_flight=1 when the aio_co_wake() in the AIO callback completes, then
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in_flight=0 after the bottom half starts.
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- Calling it through a bottom half would be okay too, as long as the AIO
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callback remembers to do inc_in_flight/dec_in_flight just like
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bdrv_co_yield_to_drain() and bdrv_co_drain_bh_cb() do
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A few more important cases that come to mind:
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- A coroutine that yields because of I/O is okay, with a sequence
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similar to bdrv_co_yield_to_drain().
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- A coroutine that yields with no I/O pending will correctly decrease
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in_flight to zero before yielding.
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- Calling more AIO from the callback won't overflow the counter just
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because of mutual recursion, because AIO functions always yield at
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least once before invoking the callback.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Reviewed-by: Fam Zheng <famz@redhat.com>
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Reviewed-by: Max Reitz <mreitz@redhat.com>
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Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
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(cherry picked from commit 46aaf2a566e364a62315219255099cbf1c9b990d)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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block/block-backend.c | 2 +-
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1 file changed, 1 insertion(+), 1 deletion(-)
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diff --git a/block/block-backend.c b/block/block-backend.c
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index bfd0331..b8ea286 100644
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--- a/block/block-backend.c
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+++ b/block/block-backend.c
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@@ -1341,8 +1341,8 @@ static const AIOCBInfo blk_aio_em_aiocb_info = {
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static void blk_aio_complete(BlkAioEmAIOCB *acb)
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{
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if (acb->has_returned) {
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- blk_dec_in_flight(acb->rwco.blk);
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acb->common.cb(acb->common.opaque, acb->rwco.ret);
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+ blk_dec_in_flight(acb->rwco.blk);
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qemu_aio_unref(acb);
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}
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}
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--
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1.8.3.1
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