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Date: Sun, 10 May 2020 08:20:58 +0200 From: Greg KH <gregkh@...uxfoundation.org> To: Luis Chamberlain <mcgrof@...nel.org> Cc: axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org, rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz, ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Omar Sandoval <osandov@...com>, Hannes Reinecke <hare@...e.com>, Michal Hocko <mhocko@...nel.org>, Christoph Hellwig <hch@....de> Subject: Re: [PATCH v4 1/5] block: revert back to synchronous request_queue removal 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
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