linux-kernel - Re: [patch 2/2]blk-mq: Don't reserve a tag for flush request

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Message-ID: <52C2ECF1.3020808@kernel.dk>
Date:	Tue, 31 Dec 2013 09:12:33 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Shaohua Li <shli@...nel.org>, linux-kernel@...r.kernel.org
CC:	hch@...radead.org, kmo@...erainc.com
Subject: Re: [patch 2/2]blk-mq: Don't reserve a tag for flush request

On 12/30/2013 08:38 PM, Shaohua Li wrote:
> 
> Reserving a tag (request) for flush to avoid dead lock is a overkill. tag is
> valuable resource. We can track flush request number and disallow too many
> pending flush requests allocated. With this patch,
> blk_mq_alloc_request_pinned() could do a busy nop (but not a dead loop) if too
> pending requests are allocated and new flush request is allocating. But this
> should not be a problem, too many pending flush requests are very rare case.
> 
> I verified this can fix the deadlock caused by too many pending flush requests.

I think this looks pretty good, and it's a lot better than the hack that
is currently in place. The atomic inc/dec should be a noop on a per-hctx
basis, especially when it's only per-flush...

I'll get this queued up, with the ida fix too.

-- 
Jens Axboe

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