Re: [PATCH] ratelimit: protect with a mutex

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

13.04.2021 15:55, Paolo Bonzini wrote:
> Right now, rate limiting is protected by the AioContext mutex, which is
> taken for example both by the block jobs and by qmp_block_job_set_speed
> (via find_block_job).
> 
> We would like to remove the dependency of block layer code on the
> AioContext mutex, since most drivers and the core I/O code are already
> not relying on it.  However, there is no existing lock that can easily
> be taken by both ratelimit_set_speed and ratelimit_calculate_delay,
> especially because the latter might run in coroutine context (and
> therefore under a CoMutex) but the former will not.
> 
> Since concurrent calls to ratelimit_calculate_delay are not possible,
> one idea could be to use a seqlock to get a snapshot of slice_ns and
> slice_quota.  But for now keep it simple, and just add a mutex to the
> RateLimit struct; block jobs are generally not performance critical to
> the point of optimizing the clock cycles spent in synchronization.
> 
> This also requires the introduction of init/destroy functions, so
> add them to the two users of ratelimit.h.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>


Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>


-- 
Best regards,
Vladimir