linux-kernel - Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount completion

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Message-ID: <20170921171644.GA3813@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Thu, 21 Sep 2017 10:16:44 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Theodore Ts'o <tytso@....edu>, Al Viro <viro@...IV.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount
 completion

On 09/21, Theodore Ts'o wrote:
> On Wed, Sep 20, 2017 at 10:02:37PM -0700, Jaegeuk Kim wrote:
> > 
> > Yes, and actually, android retries umount(2) for several seconds, if it gets
> > failure. So, first I thought it'd be better to make umount() more deterministic.
> > 
> > I'm not sure how many times we can retry and wait for this. IMHO, it'd be better
> > to use this together with the new flag, since this can detect unclosed namespace
> > given successful umount(2).
> 
> Even if you could make umount wait for the binder deferred workqueue
> to exit, if it takes time, it takes time.  And that's not the core
> VFS's fault.  It's binder's.
> 
> So... why not fix this by adding a binder ioctl which forces its
> workqueue to be flushed, and use that *before* you try calling mount?

Yes, we can, but I'm not 100% sure this happens only by binder. This is caused
by delayed_fput/mntput called by any kernel thread which would be much generic.

> If that binder ioctl takes minutes or hours to complete, then that's a
> binder bug, and needs to be fixed in binder.
> 
>        	    	      	    	  - Ted