linux-kernel - Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context

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Date:	Wed, 20 Dec 2006 20:17:33 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
Cc:	'Kiyoshi Ueda' <k-ueda@...jp.nec.com>, agk@...hat.com,
	mchristi@...hat.com, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, j-nomura@...jp.nec.com
Subject: Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context

On Wed, Dec 20 2006, Chen, Kenneth W wrote:
> Kiyoshi Ueda wrote on Wednesday, December 20, 2006 9:50 AM
> > On Wed, 20 Dec 2006 14:48:49 +0100, Jens Axboe <jens.axboe@...cle.com> wrote:
> > > Big NACK on this - it's not only really ugly, it's also buggy to pass
> > > interrupt flags as function arguments. As you also mention in the 0/1
> > > mail, this also breaks CFQ.
> > > 
> > > Why do you need in-interrupt request allocation?
> >  
> > Because I'd like to use blk_get_request() in q->request_fn()
> > which can be called from interrupt context like below:
> >   scsi_io_completion -> scsi_end_request -> scsi_next_command
> >   -> scsi_run_queue -> blk_run_queue -> q->request_fn
> > 
> > [ ...]
> > 
> > Do you think creating another function like blk_get_request_nowait()
> > is acceptable?
> 
> You don't need to create another function.  blk_get_request already
> have both wait and nowait semantics via gfp_mask argument. If you can
> not block, then clear __GFP_WAIT bit in the mask before calling
> blk_get_request.

Doesn't work, get_request() assumes that the caller grabbed the queue
lock and disabled interrupts, and does an unconditionaly

        spin_unlock_irq()

inside it. So you can NEVER use get_request() for even GFP_ATOMIC
allocations, as it assumes the original context was a process context.

-- 
Jens Axboe

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