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From: Ming Lei <ming.lei@redhat.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	Hannes Reinecke <hare@suse.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH V2 4/6] blk-mq: don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs()
Date: Mon, 26 Aug 2019 10:25:31 +0800	[thread overview]
Message-ID: <20190826022529.GB25756@ming.t460p> (raw)
In-Reply-To: <51cf9611-c62f-3dcf-2447-d8bde28bc238@acm.org>

On Wed, Aug 21, 2019 at 08:56:36AM -0700, Bart Van Assche wrote:
> On 8/21/19 2:15 AM, Ming Lei wrote:
> > blk_mq_realloc_hw_ctxs() is called from blk_mq_init_allocated_queue()
> > and blk_mq_update_nr_hw_queues(). For the former caller, the kobject
> > isn't exposed to userspace yet. For the latter caller, sysfs/debugfs
> > is un-registered before updating nr_hw_queues.
> > 
> > On the other hand, commit 2f8f1336a48b ("blk-mq: always free hctx after
> > request queue is freed") moves freeing hctx into queue's release
> > handler, so there won't be race with queue release path too.
> > 
> > So don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs().
> 
> How about mentioning that the locking at the start of
> blk_mq_update_nr_hw_queues() serializes all blk_mq_realloc_hw_ctxs() calls
> that happen after a queue has been registered in sysfs?

This patch is actually wrong because elevator switch still may happen
during updating nr_hw_queues, since only hctx sysfs entries are
un-registered, and "queue/scheduler" is still visible to userspace.

So I will drop this patch in V3.

Thanks,
Ming

  reply	other threads:[~2019-08-26  2:25 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-21  9:15 [PATCH V2 0/6] block: don't acquire .sysfs_lock before removing mq & iosched kobjects Ming Lei
2019-08-21  9:15 ` [PATCH V2 1/6] block: Remove blk_mq_register_dev() Ming Lei
2019-08-21  9:15 ` [PATCH V2 2/6] block: don't hold q->sysfs_lock in elevator_init_mq Ming Lei
2019-08-21 15:51   ` Bart Van Assche
2019-08-21  9:15 ` [PATCH V2 3/6] blk-mq: don't hold q->sysfs_lock in blk_mq_map_swqueue Ming Lei
2019-08-21 15:53   ` Bart Van Assche
2019-08-26  2:11     ` Ming Lei
2019-08-21  9:15 ` [PATCH V2 4/6] blk-mq: don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs() Ming Lei
2019-08-21 15:56   ` Bart Van Assche
2019-08-26  2:25     ` Ming Lei [this message]
2019-08-21  9:15 ` [PATCH V2 5/6] block: add helper for checking if queue is registered Ming Lei
2019-08-21 15:57   ` Bart Van Assche
2019-08-21  9:15 ` [PATCH V2 6/6] block: split .sysfs_lock into two locks Ming Lei
2019-08-21 16:18   ` Bart Van Assche
2019-08-22  1:28     ` Ming Lei
2019-08-22 19:52       ` Bart Van Assche
2019-08-23  1:08         ` Ming Lei
2019-08-23 16:36           ` Bart Van Assche
2019-08-23 16:46   ` Bart Van Assche
2019-08-23 22:49     ` Ming Lei

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