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@xxxxxxx>
> > 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@xxxxxxx>
> > ---
> > 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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