[Qemu-devel] [PATCH] Revert "qed: Implement .bdrv_drain"

[Qemu-devel] [PATCH] Revert "qed: Implement .bdrv_drain"

[Stefan Hajnoczi]

This reverts commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01.

Note that commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01 included some
unrelated hunks, possibly due to a merge failure or an overlooked
squash.  This only reverts the qed .bdrv_drain() implementation.

The qed .bdrv_drain() implementation is unsafe and can lead to a double
request completion.

Paolo Bonzini reports:
"The problem is that bdrv_qed_drain calls qed_plug_allocating_write_reqs
unconditionally, but this is not correct if an allocating write is
queued.  In this case, qed_unplug_allocating_write_reqs will restart the
allocating write and possibly cause it to complete.  The aiocb however
is still in use for the L2/L1 table writes, and will then be completed
again as soon as the table writes are stable."

For QEMU 2.6 we can simply revert this commit.  A full solution for the
qed need check timer may be added if the bdrv_drain() implementation is
extended.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/qed.c | 13 -------------
 1 file changed, 13 deletions(-)

Passes ./check -qed.

diff --git a/block/qed.c b/block/qed.c
index 404be1e..3da8021 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -376,18 +376,6 @@ static void bdrv_qed_attach_aio_context(BlockDriverState *bs,
     }
 }
 
-static void bdrv_qed_drain(BlockDriverState *bs)
-{
-    BDRVQEDState *s = bs->opaque;
-
-    /* Cancel timer and start doing I/O that were meant to happen as if it
-     * fired, that way we get bdrv_drain() taking care of the ongoing requests
-     * correctly. */
-    qed_cancel_need_check_timer(s);
-    qed_plug_allocating_write_reqs(s);
-    bdrv_aio_flush(s->bs, qed_clear_need_check, s);
-}
-
 static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
                          Error **errp)
 {
@@ -1692,7 +1680,6 @@ static BlockDriver bdrv_qed = {
     .bdrv_check               = bdrv_qed_check,
     .bdrv_detach_aio_context  = bdrv_qed_detach_aio_context,
     .bdrv_attach_aio_context  = bdrv_qed_attach_aio_context,
-    .bdrv_drain               = bdrv_qed_drain,
 };
 
 static void bdrv_qed_init(void)
-- 
2.5.0

Re: [Qemu-devel] [PATCH] Revert "qed: Implement .bdrv_drain"

[Paolo Bonzini]

On 08/03/2016 11:11, Stefan Hajnoczi wrote:
> This reverts commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01.
> 
> Note that commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01 included some
> unrelated hunks, possibly due to a merge failure or an overlooked
> squash.  This only reverts the qed .bdrv_drain() implementation.
> 
> The qed .bdrv_drain() implementation is unsafe and can lead to a double
> request completion.
> 
> Paolo Bonzini reports:
> "The problem is that bdrv_qed_drain calls qed_plug_allocating_write_reqs
> unconditionally, but this is not correct if an allocating write is
> queued.  In this case, qed_unplug_allocating_write_reqs will restart the
> allocating write and possibly cause it to complete.  The aiocb however
> is still in use for the L2/L1 table writes, and will then be completed
> again as soon as the table writes are stable."
> 
> For QEMU 2.6 we can simply revert this commit.  A full solution for the
> qed need check timer may be added if the bdrv_drain() implementation is
> extended.
> 
> Reported-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo

> ---
>  block/qed.c | 13 -------------
>  1 file changed, 13 deletions(-)
> 
> Passes ./check -qed.
> 
> diff --git a/block/qed.c b/block/qed.c
> index 404be1e..3da8021 100644
> --- a/block/qed.c
> +++ b/block/qed.c
> @@ -376,18 +376,6 @@ static void bdrv_qed_attach_aio_context(BlockDriverState *bs,
>      }
>  }
>  
> -static void bdrv_qed_drain(BlockDriverState *bs)
> -{
> -    BDRVQEDState *s = bs->opaque;
> -
> -    /* Cancel timer and start doing I/O that were meant to happen as if it
> -     * fired, that way we get bdrv_drain() taking care of the ongoing requests
> -     * correctly. */
> -    qed_cancel_need_check_timer(s);
> -    qed_plug_allocating_write_reqs(s);
> -    bdrv_aio_flush(s->bs, qed_clear_need_check, s);
> -}
> -
>  static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
>                           Error **errp)
>  {
> @@ -1692,7 +1680,6 @@ static BlockDriver bdrv_qed = {
>      .bdrv_check               = bdrv_qed_check,
>      .bdrv_detach_aio_context  = bdrv_qed_detach_aio_context,
>      .bdrv_attach_aio_context  = bdrv_qed_attach_aio_context,
> -    .bdrv_drain               = bdrv_qed_drain,
>  };
>  
>  static void bdrv_qed_init(void)
> 

Re: [Qemu-devel] [PATCH] Revert "qed: Implement .bdrv_drain"

[Stefan Hajnoczi]

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On Tue, Mar 08, 2016 at 10:11:16AM +0000, Stefan Hajnoczi wrote:
> This reverts commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01.
> 
> Note that commit df9a681dc9ad41c9cdeb9ecc5d060ba9abd27e01 included some
> unrelated hunks, possibly due to a merge failure or an overlooked
> squash.  This only reverts the qed .bdrv_drain() implementation.
> 
> The qed .bdrv_drain() implementation is unsafe and can lead to a double
> request completion.
> 
> Paolo Bonzini reports:
> "The problem is that bdrv_qed_drain calls qed_plug_allocating_write_reqs
> unconditionally, but this is not correct if an allocating write is
> queued.  In this case, qed_unplug_allocating_write_reqs will restart the
> allocating write and possibly cause it to complete.  The aiocb however
> is still in use for the L2/L1 table writes, and will then be completed
> again as soon as the table writes are stable."
> 
> For QEMU 2.6 we can simply revert this commit.  A full solution for the
> qed need check timer may be added if the bdrv_drain() implementation is
> extended.
> 
> Reported-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/qed.c | 13 -------------
>  1 file changed, 13 deletions(-)
> 
> Passes ./check -qed.

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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