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List:       linux-btrfs
Subject:    Re: [PATCH RESEND v4 6/8] vfs: Add get_vfsmount_sb() function to get vfsmount from a given sb.
From:       Al Viro <viro () ZenIV ! linux ! org ! uk>
Date:       2015-01-30 6:15:52
Message-ID: 20150130061550.GJ29656 () ZenIV ! linux ! org ! uk
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On Fri, Jan 30, 2015 at 01:34:41PM +0800, Miao Xie wrote:
> > 	First of all, ->s_umount is not a mutex; it's rwsem.  So you mean
> > down_read_trylock().  As for the transient failures - grep for down_write
> > on it...  E.g. have somebody call mount() from the same device.  We call
> > sget(), which finds existing superblock and calls grab_super().  Sure,
> > that ->s_umount will be released shortly, but in the meanwhile your trylock
> > will fail...
> 
> I know it, so I suggested to return -EBUSY in the previous mail.
> I think it is acceptable method, mount/umount operations are not so many
> after all.

Er...  What for, when there's a trivial variant that doesn't suffer those
spurious -EBUSY?  Seriously, just move sysfs removals up to make sure
that any possible fs shutdown won't progress past those during sysfs IO and
use sb_want_write/sb_dont_write to make sure it'll stay r/w for the duration.
What's the problem?
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