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Re: [PATCH v12 18/21] job.c: enable job lock/unlock and remove Aiocontex


From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v12 18/21] job.c: enable job lock/unlock and remove Aiocontext locks
Date: Mon, 26 Sep 2022 15:24:13 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0

On 9/26/22 12:32, Emanuele Giuseppe Esposito wrote:
Change the job_{lock/unlock} and macros to use job_mutex.

Now that they are not nop anymore, remove the aiocontext
to avoid deadlocks.

Therefore:
- when possible, remove completely the aiocontext lock/unlock pair
- if it is used by some other function too, reduce the locking
   section as much as possible, leaving the job API outside.
- change AIO_WAIT_WHILE in AIO_WAIT_WHILE_UNLOCKED, since we
   are not using the aiocontext lock anymore

The only functions that still need the aiocontext lock are:
- the JobDriver callbacks, already documented in job.h
- job_cancel_sync() in replication.c is called with aio_context_lock
   taken, but now job is using AIO_WAIT_WHILE_UNLOCKED so we need to
   release the lock.

Reduce the locking section to only cover the callback invocation
and document the functions that take the AioContext lock,
to avoid taking it twice.

Also remove real_job_{lock/unlock}, as they are replaced by the
public functions.

Signed-off-by: Emanuele Giuseppe Esposito<eesposit@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

I'm still not sure about aio_poll() call inside dropped aio-context 
acquire/release pair in run_block_job(), so that's up to other maintainers. 
Anyway I don't expect some real locking/racing problems in qemu-img.

--
Best regards,
Vladimir



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