Re: [PATCH -next v4] blk-mq: fix tag_get wait task can't be awakened

[John Garry]

On 12/01/2022 12:30, Andy Shevchenko wrote:
> > > > +		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) ||
> > > > +		    test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags)) {
> > > Whoever wrote this code did too much defensive programming, because the first
> > > conditional doesn't make much sense here. Am I right?
> > I think because this judgement is in the general IO process, there are also
> > some performance considerations here.
> I didn't buy this. Is there any better argument why you need redundant
> test_bit() call?

I think that the idea is that test_bit() is fast and test_and_set_bit() 
is slow; as such, if we generally expect the bit to be set, then there 
is no need to do the slower test_and_set_bit() always.