Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO scheduler

[Paolo Valente]

> Il giorno 02 feb 2017, alle ore 16:30, Jens Axboe <axboe@xxxxxx> ha scritto:
> 
> On 02/02/2017 02:19 AM, Paolo Valente wrote:
>> The scheme is clear.  One comment, in case it could make sense and
>> avoid more complexity: since put_rq_priv is invoked in two different
>> contexts, process or interrupt, I didn't feel so confusing that, when
>> put_rq_priv is invoked in the context where the lock cannot be held
>> (unless one is willing to pay with irq disabling all the times), the
>> lock is not held, while, when invoked in the context where the lock
>> can be held, the lock is actually held, or must be taken.
> 
> If you grab the same lock from put_rq_priv, yes, you must make it IRQ
> disabling in all contexts, and use _irqsave() from put_rq_priv. If it's
> just freeing resources, you could potentially wait and do that when
> someone else needs them, since that part will come from proces context.
> That would need two locks, though.
> 
> As I said above, I would not worry about the IRQ disabling lock.
> 

I'm sorry, I focused only on the IRQ-disabling consequence of grabbing
a scheduler lock also in IRQ context.  I thought it was a serious
enough issue to avoid this option.  Yet there is also a deadlock
problem related to this option.  In fact, the IRQ handler may preempt
some process-context code that already holds some other locks, and, if
some of these locks are already held by another process, which is
executing on another CPU and which then tries to take the scheduler
lock, or which happens to be preempted by an IRQ handler trying to
grab the scheduler lock, then a deadlock occurs.  This is not just a
speculation, but a problem that did occur before I moved to a
deferred-work solution, and that can be readily reproduced.  Before
moving to a deferred work solution, I tried various code manipulations
to avoid these deadlocks without resorting to deferred work, but at no
avail.

At any rate, bfq seems now to work, so I can finally move from just
asking questions endlessly, to proposing actual code to discuss on.
I'm about to: port this version of bfq to your improved/fixed
blk-mq-sched version in for-4.11 (port postponed, to avoid introducing
further changes in code that did not yet wok), run more extensive
tests, polish commits a little bit, and finally share a branch.

Thanks,
Paolo

> -- 
> Jens Axboe
> 
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