linux-kernel - Re: [PATCH v3] loop: Limit the number of requests in the bio list

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Message-ID: <50A27892.1030800@kernel.dk>
Date:	Tue, 13 Nov 2012 09:42:58 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	jmoyer@...hat.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v3] loop: Limit the number of requests in the bio list

> @@ -489,6 +491,12 @@ static void loop_make_request(struct request_queue *q, struct bio *old_bio)
>  		goto out;
>  	if (unlikely(rw == WRITE && (lo->lo_flags & LO_FLAGS_READ_ONLY)))
>  		goto out;
> +	if (lo->lo_bio_count >= q->nr_congestion_on) {
> +		spin_unlock_irq(&lo->lo_lock);
> +		wait_event(lo->lo_req_wait, lo->lo_bio_count <
> +			   q->nr_congestion_off);
> +		spin_lock_irq(&lo->lo_lock);
> +	}

This makes me nervous. You are reading lo_bio_count outside the lock. If
you race with the prepare_to_wait() and condition check in
__wait_event(), then you will sleep forever.

md has private helpers for this, seems it would be a good idea to move
these into the regular wait includes and use them here too.

-- 
Jens Axboe

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