Re: [Qemu-devel] [PATCH] blockdev: avoid acquiring AioContext lock twice at do_drive_backup()

[Max Reitz <mreitz@redhat.com>]

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On 13.09.19 11:37, Sergio Lopez wrote:
> 
> Max Reitz <mreitz@redhat.com> writes:
> 
>> On 12.09.19 18:16, Sergio Lopez wrote:
>>> do_drive_backup() acquires the AioContext lock of the corresponding
>>> BlockDriverState. This is not a problem when it's called from
>>> qmp_drive_backup(), but drive_backup_prepare() also acquires the lock
>>> before calling it.
>>>
>>> This change adds a BlockDriverState argument to do_drive_backup(),
>>> which is used to signal that the context lock is already acquired and
>>> to save a couple of redundant calls.
>>
>> But those redundant calls don’t really hurt (it’s just bdrv_lookup_bs(),
>> as far as I can tell).  Wouldn’t it be simpler to just release the
>> context lock in drive_backup_prepare() before calling do_drive_backup()?
>>  The BDS is drained anyway.
> 
> Redundant calls rarely hurt, they're just redundant ;-)

If they’re expensive and in a hot path, they hurt.

>> On top of that, do_backup_common() calls bdrv_try_set_aio_context() to
>> bring the target into the source’s AioContext.  However, this function
>> must be called with the old AioContext held, and the new context not held.
> 
> Is this documented somewhere? I see nothing in the function declaration
> nor definition.
> 
> I'm starting to get the feeling that the block layer is riddled with
> unwritten rules and assumptions that makes every change a lot harder
> than it should be.

It is written, it’s just that it’s written in
bdrv_set_aio_context_ignore()’s definition.

Yes, we should document it directly for bdrv_try_set_aio_context(), too,
because that’s what external callers are much more likely to use.

>> Currently, it’s called exactly the other way around: With the new
>> context held, but the old one not held.
>>
>> So I think it indeed actually makes more sense to release the AioContext
>> before calling do_drive_backup(), and to move the
>> bdrv_try_set_aio_context() call for target_bs to the callers of
>> do_backup_common() (where they have not yet taken the AioContext lock).
> 
> OK. I see this also happens in external_snapshot_prepare() and
> qmp_drive_mirror() too. I guess we should fix these too.
> 
> In qmp_drive_mirror(), would it be safe to delay the acquisition of any
> context until just before the blockdev_mirror_common()?

From mirror’s perspective I think so, but I don’t think it’s safe to
access any of a BDS’s fields without having acquired its AioContext.
(In fact, I wonder whether we should acquire the context even before
bdrv_op_is_blocked()...)

Max


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