On 8/11/25 13:25, Yu Kuai wrote:

Hi,

在 2025/08/11 11:53, Damien Le Moal 写道:

quoted

On 8/11/25 10:01, Yu Kuai wrote:

quoted

quoted

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diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 55a0fd105147..1a2da5edbe13 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c

@@ -113,7 +113,14 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
   		if (budget_token < 0)
   			break;
   
-		rq = e->type->ops.dispatch_request(hctx);
+		if (blk_queue_sq_sched(q)) {
+			elevator_lock(e);
+			rq = e->type->ops.dispatch_request(hctx);
+			elevator_unlock(e);

I do not think this is safe for bfq since bfq uses the irqsave/irqrestore spin
lock variant. If it is safe, this needs a big comment block explaining why
and/or the rules regarding the scheduler use of this lock.

It's correct, however, this patch doesn't change bfq yet, and it's like:

elevator_lock
spin_lock_irq(&bfqd->lock)
spin_unlock_irq(&bfqd->lock)
elevator_unlock

Patch 3 remove bfqd->lock and convert this to:

elevator_lock_irq
elevator_unlock_irq.

I do not understand. Since q->elevator->lock is already taken here, without IRQ
disabled, how can bfq_dispatch_request method again take this same lock with IRQ
disabled ? That cannot possibly work.

Looks like there is still misunderstanding somehow :( After patch 3,
bfq_dispatch_work doesn't grab any lock, elevator lock is held before
calling into dispatch method.

Before:

elevator_lock
bfq_dispatch_request
  spin_lock_irq(&bfqd->lock)
  spin_unlock_irq(&bfqd->lock)
elevator_unlock

After:
elevator_lock_irq
bfq_dispatch_request
elevator_unlock_irq

Ah, yes, I see it now.

But that is a nasty change that affects *all* schedulers, even those that do not
need to disable IRQs because they are not using the lock in their completion
path, e.g. mq-deadline. So I do not think that is acceptable.

-- 
Damien Le Moal
Western Digital Research