Hi,
在 2025/08/11 11:53, Damien Le Moal 写道:
On 8/11/25 10:01, Yu Kuai wrote:
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
The other side of this is that if you use elevator_lock(e) to call
e->type->ops.dispatch_request(hctx), it means that the scheduler *can NOT* use
that same lock in its completion path, since that can be called from IRQ
context. This may not be needed/a problem right now, but I think this needs a
comment in this patch to mention this.
So, the first patch just grab elevator lock for dispatch method, bfq
dispatch still using spin_lock_irq(&bfqd->lock), and complete is still
using bfqd->lock, this is safe.
Later patch 3 remove bfqd->lock and use elevator lock instead, and
because elevator lock will be used in complete path now, elevator_lock
from dispatch path is converted to elevator_lock_irq.
Thanks,
Kuai
