Re: [PATCH] gfs2: Get rid of I_MUTEX_QUOTA usage

[Steven Whitehouse <swhiteho@xxxxxxxxxx>]

Hi,

On Thu, 2012-04-26 at 15:47 +0200, Jan Kara wrote:
> Hello,
> 
> On Thu 26-04-12 14:28:39, Steven Whitehouse wrote:
> > On Thu, 2012-04-26 at 15:20 +0200, Jan Kara wrote:
> > > GFS2 uses i_mutex on its system quota inode to synchronize writes to
> > > quota file. Since this is an internal inode to GFS2 (not part of directory
> > > hiearchy or visible by user) we are safe to define locking rules for it. So
> > > let's just get it its own locking class to make it clear.
> > > 
> > It is visible to the user if the gfs2 metadata filesystem is mounted.
> > That used to be the way in which the quotas were set by gfs2_quota.
> > However that is really considered obsolete now, so maybe we don't have
> > to support that interface any more... the gfs2_quota package is no
> > longer in distros as the generic quota package does the job for us these
> > days.
>   Ah, OK. But then if users could write to (or even truncate?) the quota
> file, wasn't it really deadlockable? mutex_lock_nested would silence
> lockdep but the deadlock won't change. Another advantage of my change is
> that if there is some problem, lockdep will warn about it because all
> places which end up taking i_mutex on quota file will use the same locking
> class. Using of lockdep subclasses (i.e. the _nested variant of mutex_lock)
> essentially tells lockdep - I promise that uses of the lock tagged as
> one locking subclass cannot interact with uses tagged as another locking
> class. 
> 
> 								Honza
> 
It has been fairly well tested over a period of time, so I'm pretty
confident that it does work at the moment. The locking is not that
obvious though and could certainly do with a clean up. I'd prefer to
clean up the locking than just silence the messages.

I am taking a look at it currently to see what we might do. It may take
me a little while to get something together though, bearing in mind all
the possible cases that need checking. If we can use sd_quota_mutex
everywhere, then that should at least reduce the complexity and remove
the need for I_MUTEX_QUOTA, so thats my preferred solution if possible.
Otherwise I'm open to suggestions,

Steve.

> > > CC: Steven Whitehouse <swhiteho@xxxxxxxxxx>
> > > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > > ---
> > >  fs/gfs2/ops_fstype.c |    8 ++++++++
> > >  fs/gfs2/quota.c      |    2 +-
> > >  2 files changed, 9 insertions(+), 1 deletions(-)
> > > 
> > >  So this is probably the simplest what can be done for GFS2 (actually OCFS2
> > > does the same for internal system files). Compile tested only. Steven?
> > > 
> > > diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
> > > index 6f3a18f..ae8f225 100644
> > > --- a/fs/gfs2/ops_fstype.c
> > > +++ b/fs/gfs2/ops_fstype.c
> > > @@ -19,6 +19,7 @@
> > >  #include <linux/mount.h>
> > >  #include <linux/gfs2_ondisk.h>
> > >  #include <linux/quotaops.h>
> > > +#include <linux/lockdep.h>
> > >  
> > >  #include "gfs2.h"
> > >  #include "incore.h"
> > > @@ -767,6 +768,7 @@ fail:
> > >  	return error;
> > >  }
> > >  
> > > +static struct lock_class_key gfs2_quota_imutex_key;
> > >  
> > >  static int init_inodes(struct gfs2_sbd *sdp, int undo)
> > >  {
> > > @@ -804,6 +806,12 @@ static int init_inodes(struct gfs2_sbd *sdp, int undo)
> > >  		fs_err(sdp, "can't get quota file inode: %d\n", error);
> > >  		goto fail_rindex;
> > >  	}
> > > +	/*
> > > +	 * i_mutex on quota files is special. Since this inode is hidden system
> > > +	 * file, we are safe to define locking ourselves.
> > > +	 */
> > > +	lockdep_set_class(&sdp->sd_quota_inode->i_mutex,
> > > +			  &gfs2_quota_imutex_key);
> > >  
> > >  	error = gfs2_rindex_update(sdp);
> > >  	if (error)
> > > diff --git a/fs/gfs2/quota.c b/fs/gfs2/quota.c
> > > index 6019da3..970598b 100644
> > > --- a/fs/gfs2/quota.c
> > > +++ b/fs/gfs2/quota.c
> > > @@ -772,7 +772,7 @@ static int do_sync(unsigned int num_qd, struct gfs2_quota_data **qda)
> > >  		return -ENOMEM;
> > >  
> > >  	sort(qda, num_qd, sizeof(struct gfs2_quota_data *), sort_qd, NULL);
> > > -	mutex_lock_nested(&ip->i_inode.i_mutex, I_MUTEX_QUOTA);
> > > +	mutex_lock(&ip->i_inode.i_mutex);
> > >  	for (qx = 0; qx < num_qd; qx++) {
> > >  		error = gfs2_glock_nq_init(qda[qx]->qd_gl, LM_ST_EXCLUSIVE,
> > >  					   GL_NOCACHE, &ghs[qx]);
> > 
> > 


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