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From: Greg KH <gregkh@linuxfoundation.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org,
	rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz,
	ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org,
	mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Omar Sandoval <osandov@fb.com>,
	Hannes Reinecke <hare@suse.com>, Michal Hocko <mhocko@kernel.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v4 1/5] block: revert back to synchronous request_queue removal
Date: Sun, 10 May 2020 08:20:58 +0200	[thread overview]
Message-ID: <20200510062058.GA3394360@kroah.com> (raw)
In-Reply-To: <20200509031058.8239-2-mcgrof@kernel.org>

On Sat, May 09, 2020 at 03:10:54AM +0000, Luis Chamberlain wrote:
> Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
> v4.12 moved the work behind blk_release_queue() into a workqueue after a
> splat floated around which indicated some work on blk_release_queue()
> could sleep in blk_exit_rl(). This splat would be possible when a driver
> called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
> as its final call) from an atomic context.
> 
> blk_put_queue() decrements the refcount for the request_queue kobject,
> and upon reaching 0 blk_release_queue() is called. Although blk_exit_rl()
> is now removed through commit db6d9952356 ("block: remove request_list code")
> on v5.0, we reserve the right to be able to sleep within blk_release_queue()
> context.
> 
> The last reference for the request_queue must not be called from atomic
> context. *When* the last reference to the request_queue reaches 0 varies,
> and so let's take the opportunity to document when that is expected to
> happen and also document the context of the related calls as best as possible
> so we can avoid future issues, and with the hopes that the synchronous
> request_queue removal sticks.
> 
> We revert back to synchronous request_queue removal because asynchronous
> removal creates a regression with expected userspace interaction with
> several drivers. An example is when removing the loopback driver, one
> uses ioctls from userspace to do so, but upon return and if successful,
> one expects the device to be removed. Likewise if one races to add another
> device the new one may not be added as it is still being removed. This was
> expected behavior before and it now fails as the device is still present
> and busy still. Moving to asynchronous request_queue removal could have
> broken many scripts which relied on the removal to have been completed if
> there was no error. Document this expectation as well so that this
> doesn't regress userspace again.
> 
> Using asynchronous request_queue removal however has helped us find
> other bugs. In the future we can test what could break with this
> arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.

You are adding documenation and might_sleep() calls all over the place
in here, making the "real" change in the patch hard to pick out.

How about you split this up into 3 patches, one for documentation, one
for might_sleep() and one for the real change?  Or maybe just 2 patches,
but what you have here seems excessive.

thanks,

greg k-h

  parent reply	other threads:[~2020-05-10  6:21 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-09  3:10 [PATCH v4 0/5] block: fix blktrace debugfs use after free Luis Chamberlain
2020-05-09  3:10 ` [PATCH v4 1/5] block: revert back to synchronous request_queue removal Luis Chamberlain
2020-05-10  0:36   ` Bart Van Assche
2020-05-10  6:20   ` Greg KH [this message]
2020-05-11 13:41     ` Luis Chamberlain
2020-05-09  3:10 ` [PATCH v4 2/5] block: move main block debugfs initialization to its own file Luis Chamberlain
2020-05-09  3:10 ` [PATCH v4 3/5] blktrace: fix debugfs use after free Luis Chamberlain
2020-05-10  0:58   ` Bart Van Assche
2020-05-11 13:44     ` Luis Chamberlain
2020-05-10  6:26   ` Greg KH
2020-05-11 14:03     ` Luis Chamberlain
2020-05-09  3:10 ` [PATCH v4 4/5] blktrace: break out of blktrace setup on concurrent calls Luis Chamberlain
2020-05-10  1:09   ` Bart Van Assche
2020-05-11 13:39     ` Luis Chamberlain
2020-05-16  1:39       ` Luis Chamberlain
2020-05-16  1:39         ` Luis Chamberlain
2020-05-09  3:10 ` [PATCH v4 5/5] loop: be paranoid on exit and prevent new additions / removals Luis Chamberlain

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