Re: [Qemu-devel] [PATCH] file-posix: Cache lseek result for data regions

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/01/19 11:30, Kevin Wolf wrote:
>> On the other hand, the AioContext lock is only used in
>> some special cases around block jobs and blk_set_aio_context, and in
>> general the block devices already should not have any dependencies
>> (unless they crept in without me noticing).
> It's also used in those cases where coroutines don't need locking, but
> threads would. Did you audit all of the drivers for such cases?

I did and the drivers already have a QemuMutex if that's the case (e.g.
curl, iscsi).

>> In particular...
>>
>>> But raw doesn't have an s->lock yet, so I
>>> think removing the AioContext lock involves some work on it anyway and
>>> adding this doesn't really change the amount of work.
>> ... BDRVRawState doesn't have any data that changes after open, does it?
>>  This is why it doesn't have an s->lock.
> No important data anyway. We do things like setting s->has_write_zeroes
> = false after failure, but if we got a race and end up trying twice
> before disabling it, it doesn't really hurt either.
> 
> Then there is reopen, but that involves a drain anyway. And that's it
> probably.
> 
> So do you think I should introduce a CoMutex for raw here? Or QemuMutex?

For the cache you can introduce either a CoMutex or QemuMutex, it's the
same.

Paolo