[Qemu-commits] [qemu/qemu] ce83ee: block: fix deadlock in bdrv_co_flush

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/qemu/qemu
  Commit: ce83ee57f6fa69280a0330f1b49ef75b0ef0742a
      
https://github.com/qemu/qemu/commit/ce83ee57f6fa69280a0330f1b49ef75b0ef0742a
  Author: Evgeny Yakovlev <address@hidden>
  Date:   2016-08-18 (Thu, 18 Aug 2016)

  Changed paths:
    M block/io.c
    M include/block/block_int.h

  Log Message:
  -----------
  block: fix deadlock in bdrv_co_flush

The following commit
    commit 3ff2f67a7c24183fcbcfe1332e5223ac6f96438c
    Author: Evgeny Yakovlev <address@hidden>
    Date:   Mon Jul 18 22:39:52 2016 +0300
    block: ignore flush requests when storage is clean
has introduced a regression.

There is a problem that it is still possible for 2 requests to execute
in non sequential fashion and sometimes this results in a deadlock
when bdrv_drain_one/all are called for BDS with such stalled requests.

1. Current flushed_gen and flush_started_gen is 1.
2. Request 1 enters bdrv_co_flush to with write_gen 1 (i.e. the same
   as flushed_gen). It gets past flushed_gen != flush_started_gen and
   sets flush_started_gen to 1 (again, the same it was before).
3. Request 1 yields somewhere before exiting bdrv_co_flush
4. Request 2 enters bdrv_co_flush with write_gen 2. It gets past
   flushed_gen != flush_started_gen and sets flush_started_gen to 2.
5. Request 2 runs to completion and sets flushed_gen to 2
6. Request 1 is resumed, runs to completion and sets flushed_gen to 1.
   However flush_started_gen is now 2.

>From here on out flushed_gen is always != to flush_started_gen and all
further requests will wait on flush_queue. This change replaces
flush_started_gen with an explicitly tracked active flush request.

Signed-off-by: Evgeny Yakovlev <address@hidden>
Signed-off-by: Denis V. Lunev <address@hidden>
Message-id: address@hidden
CC: Stefan Hajnoczi <address@hidden>
CC: Fam Zheng <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Max Reitz <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>


  Commit: 156af3ac98da24f0155eed18ec546157436d6b2e
      
https://github.com/qemu/qemu/commit/156af3ac98da24f0155eed18ec546157436d6b2e
  Author: Denis V. Lunev <address@hidden>
  Date:   2016-08-18 (Thu, 18 Aug 2016)

  Changed paths:
    M block/io.c

  Log Message:
  -----------
  block: fix possible reorder of flush operations

This patch reduce CPU usage of flush operations a bit. When we have one
flush completed we should kick only next operation. We should not start
all pending operations in the hope that they will go back to wait on
wait_queue.

Also there is a technical possibility that requests will get reordered
with the previous approach. After wakeup all requests are removed from
the wait queue. They become active and they are processed one-by-one
adding to the wait queue in the same order. Though new flush can arrive
while all requests are not put into the queue.

Signed-off-by: Denis V. Lunev <address@hidden>
Tested-by: Evgeny Yakovlev <address@hidden>
Signed-off-by: Evgeny Yakovlev <address@hidden>
Message-id: address@hidden
CC: Stefan Hajnoczi <address@hidden>
CC: Fam Zheng <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Max Reitz <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>


  Commit: 02b1ad881cbb1795029737a9077db60267dc0c6f
      
https://github.com/qemu/qemu/commit/02b1ad881cbb1795029737a9077db60267dc0c6f
  Author: Peter Maydell <address@hidden>
  Date:   2016-08-18 (Thu, 18 Aug 2016)

  Changed paths:
    M block/io.c
    M include/block/block_int.h

  Log Message:
  -----------
  Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into 
staging

# gpg: Signature made Thu 18 Aug 2016 14:39:31 BST
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <address@hidden>"
# gpg:                 aka "Stefan Hajnoczi <address@hidden>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  block: fix possible reorder of flush operations
  block: fix deadlock in bdrv_co_flush

Signed-off-by: Peter Maydell <address@hidden>


Compare: https://github.com/qemu/qemu/compare/5844365fe8e5...02b1ad881cbb