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Date: Tue, 26 Mar 2024 07:51:06 +0100
From: Christoph Hellwig <hch@lst.de>
To: Damien Le Moal <dlemoal@kernel.org>
Cc: Bart Van Assche <bvanassche@acm.org>, linux-block@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>, linux-scsi@vger.kernel.org,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	dm-devel@lists.linux.dev, Mike Snitzer <snitzer@redhat.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2 07/28] block: Introduce zone write plugging
Message-ID: <20240326065106.GF7986@lst.de>
References: <20240325044452.3125418-1-dlemoal@kernel.org> <20240325044452.3125418-8-dlemoal@kernel.org> <f3b298bb-b68a-4375-a3b4-fc91229740c1@acm.org> <839ebf2a-4dc6-433b-bc47-fd7915ed0ecf@kernel.org>
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On Tue, Mar 26, 2024 at 12:12:03PM +0900, Damien Le Moal wrote:
> Nope, we cannot. The reason is that BIO issuing and zone reset/finish can be
> concurrently processed and we need to be ready for a user doing really stupid
> things like resetting or finishing a zone while BIOs for that zone are being
> issued. When zone reset/finish is processed, the plug is removed from the hash
> table, but disk_get_zone_wplug_locked() may still get a reference to it because
> we do not have the plug locked yet. Hence the flag, to prevent reusing the plug
> for the reset/finished zone that was already removed from the hash table. This
> is mentioned with a comment in disk_get_zone_wplug_locked():
> 
> 	/*
> 	 * Check that a BIO completion or a zone reset or finish
> 	 * operation has not already flagged the zone write plug for
> 	 * freeing and dropped its reference count. In such case, we
> 	 * need to get a new plug so start over from the beginning.
> 	 */
> 
> The reference count dropping to 0 will then be the trigger for actually freeing
> the plug, after all in-flight or plugged BIOs are completed (most likely failed).

Maybe the comment should be expanded even more and move to the definition
of the freeing flag?