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Date: Thu, 18 Feb 2016 14:20:45 +0100 From: Jan Kara <jack@...e.cz> To: Tejun Heo <tj@...nel.org> Cc: Jan Kara <jack@...e.cz>, Tahsin Erdogan <tahsin@...gle.com>, Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org, Theodore Ts'o <tytso@....edu>, Nauman Rafique <nauman@...gle.com>, linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>, Al Viro <viro@...IV.linux.org.uk> Subject: Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned during cgroup writeback association switches Hi Tejun, On Thu 18-02-16 08:00:33, Tejun Heo wrote: > On Thu, Feb 18, 2016 at 10:55:38AM +0100, Jan Kara wrote: > > I'm not sure I understand the question. Do you mean why both s_active and > > s_umount rwsem exist? s_active is a reference count keeping superblock > > Yes. > > > alive - e.g. if the filesystem is mounted in more places, we need a > > reference for each mountpoint. s_umount is used when we want to block any > > I could be mistaken but I *think* we used to reject umounts based on > s_active and s_umount is the mechanism to delay umounts rather than > failing them and probably with bind mounts the behavior changed. > > > umount operation until we are done. For example sync(2) is using it to make > > sure superblock doesn't disappear and so that we don't keep superblock > > alive after admin called umount(2). > > So, the question is why aren't we just using s_active and draining it > on umount of the last mountpoint. Because, right now, the behavior is > weird in that we allow umounts to proceed but then let the superblock > hang onto the block device till s_active is drained. This really > should be synchronous. Hum, I'm not sure. I guess Al can give you more qualified answer than me. Added to CC... Honza -- Jan Kara <jack@...e.com> SUSE Labs, CR
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