linux-kernel - Re: [PATCH 1/2] vfs: Provide function to get superblock and wait for it to thaw

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Message-ID: <Pine.LNX.4.64.1202091811230.13859@hs20-bc2-1.build.redhat.com>
Date:	Thu, 9 Feb 2012 18:14:36 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Eric Sandeen <sandeen@...deen.net>
cc:	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] vfs: Provide function to get superblock and wait
 for it to thaw



On Thu, 9 Feb 2012, Eric Sandeen wrote:

> On 2/8/12 5:47 PM, Jan Kara wrote:
> > On Thu 09-02-12 00:27:05, Jan Kara wrote:
> >> On Thu 09-02-12 10:20:49, Dave Chinner wrote:
> 
> ...
> 
> >>> The thawed parameter appears to be unused and the waiting for frozen
> >>> filesystems appears to happen for all callers. Is this intentional?
> >>   Ah, sorry. Obviously that's a bug. Thanks for spotting it. I'll fix it
> >> up.
> >   Attached is a fixed version.
> > 
> > 								Honza
> 
> > From 5dce7adb0c281612a14fa3dd8c8d5ef3f5eb3666 Mon Sep 17 00:00:00 2001
> > From: Jan Kara <jack@...e.cz>
> > Date: Tue, 7 Feb 2012 22:59:06 +0100
> > Subject: [PATCH 1/2] vfs: Provide function to get superblock and wait for it to thaw
> > 
> > In quota code we need to find a superblock corresponding to a device and wait
> > for superblock to be unfrozen. However this waiting has to happen without
> > s_umount semaphore because that is required for superblock to thaw. So provide
> > a function in VFS for this to keep dances with s_umount where they belong.
> > 
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/super.c         |   47 +++++++++++++++++++++++++++++++++++------------
> >  include/linux/fs.h |    1 +
> >  2 files changed, 36 insertions(+), 12 deletions(-)
> > 
> > diff --git a/fs/super.c b/fs/super.c
> > index 6015c02..e15aaa9 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -593,15 +593,7 @@ void iterate_supers_type(struct file_system_type *type,
> >  
> >  EXPORT_SYMBOL(iterate_supers_type);
> >  
> > -/**
> > - *	get_super - get the superblock of a device
> > - *	@bdev: device to get the superblock for
> > - *	
> > - *	Scans the superblock list and finds the superblock of the file system
> > - *	mounted on the device given. %NULL is returned if no match is found.
> > - */
> 
> > -
> > -struct super_block *get_super(struct block_device *bdev)
> > +static struct super_block *__get_super(struct block_device *bdev, bool thawed)
> >  {
> >  	struct super_block *sb;
> >  
> > @@ -618,9 +610,13 @@ rescan:
> >  			spin_unlock(&sb_lock);
> >  			down_read(&sb->s_umount);
> >  			/* still alive? */
> > -			if (sb->s_root && (sb->s_flags & MS_BORN))
> > -				return sb;
> > -			up_read(&sb->s_umount);
> > +			if (sb->s_root && (sb->s_flags & MS_BORN)) {
> > +				if (!thawed || sb->s_frozen == SB_UNFROZEN)
> 
> I guess this has the same race we are worried about elsewhere (sb gets
> frozen immediately after we check) but probably can't fix that yet.

It doesn't --- s_umount is locked for read for the whole duration of the 
quota call, so superblock cannot be frozen (because freezing takes 
s_umount for write).

Mikulas
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