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Date: Thu, 28 Mar 2024 07:03:57 +0100
From: Christoph Hellwig <hch@lst.de>
To: Damien Le Moal <dlemoal@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, 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>,
	linux-nvme@lists.infradead.org, Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCH v3 09/30] block: Pre-allocate zone write plugs
Message-ID: <20240328060357.GA16819@lst.de>
References: <20240328004409.594888-1-dlemoal@kernel.org> <20240328004409.594888-10-dlemoal@kernel.org> <20240328043016.GA13701@lst.de> <714d0cbc-be4d-4aa9-b200-73c6caaa1d18@kernel.org> <20240328054652.GA16237@lst.de> <7d8f3ec4-c416-445f-92db-7d2b60726821@kernel.org>
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On Thu, Mar 28, 2024 at 03:02:54PM +0900, Damien Le Moal wrote:
> But that is the problem: "checking the zone number again" means referencing the
> plug struct again from the lookup context while the last ref drop context is
> freeing the plug. That race can be lost by the lookup context and lead to
> referencing freed memory. So your solution would be OK for pre-allocated plugs
> only.

Not if it is done in the Rcu critical section.

> For kmalloc-ed() plugs, we still need the rcu grace period for free. So we
> can only optimize for the pre-allocated plugs...

Yes, bt it can use kfree_rcu which doesn't need the rcu_head in the
zwplug.