Date:	Tue, 24 Mar 2009 12:30:32 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	zdenek.kabelac@...il.com, bunk@...nel.org, jirislaby@...il.com,
	hidave.darkstar@...il.com, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org, nikanth@...il.com
Subject: Re: [PATCH] Fix Bug 10504 - losetup possible circular locking

On Tue, Mar 24 2009, Nikanth Karthikesan wrote:
> Hi Jens
> 
> Did you get to look at this? Can you ACK/NACK this one?

It looks fine, I have applied it.

> 
> Thanks
> Nikanth
> 
> On Thursday 12 March 2009 13:41:12 Nikanth Karthikesan wrote:
> > With CONFIG_PROVE_LOCKING enabled
> >
> > $ losetup /dev/loop0 file
> > $ losetup -o 32256 /dev/loop1 /dev/loop0
> >
> > $ losetup -d /dev/loop1
> > $ losetup -d /dev/loop0
> >
> > triggers a [ INFO: possible circular locking dependency detected ]
> >
> > I think this warning is a false positive.
> >
> > Open/close on a loop device acquires bd_mutex of the device before
> > acquiring lo_ctl_mutex of the same device. For ioctl(LOOP_CLR_FD) after
> > acquiring lo_ctl_mutex, fput on the backing_file might acquire the bd_mutex
> > of a device, if backing file is a device and this is the last reference to
> > the file being dropped . But it is guaranteed that it is impossible to have
> > a circular list of backing devices.(say loop2->loop1->loop0->loop2 is not
> > possible), which guarantees that this can never deadlock.
> >
> > So this warning should be suppressed. It is very difficult to annotate
> > lockdep not to warn here in the correct way. A simple way to silence
> > lockdep could be to mark the lo_ctl_mutex in ioctl to be a sub class, but
> > this might mask some other real bugs.
> >
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -1164,7 +1164,7 @@ static int lo_ioctl(struct block_device *bdev,
> > fmode_t mode, struct loop_device *lo = bdev->bd_disk->private_data;
> >  	int err;
> >
> > -	mutex_lock(&lo->lo_ctl_mutex);
> > +	mutex_lock_nested(&lo->lo_ctl_mutex, 1);
> >  	switch (cmd) {
> >  	case LOOP_SET_FD:
> >  		err = loop_set_fd(lo, mode, bdev, arg);
> >
> > Or actually marking the bd_mutex after lo_ctl_mutex as a sub class could be
> > a better solution.
> >
> > Luckily it is easy to avoid calling fput on backing file with lo_ctl_mutex
> > held, so no lockdep annotation is required.
> >
> > If you do not like the special handling of the lo_ctl_mutex just for the
> > LOOP_CLR_FD ioctl in lo_ioctl(), the mutex handling could be moved inside
> > each of the individual ioctl handlers and I could send you another patch.
> >
> > Thanks
> > Nikanth Karthikesan
> >
> > Signed-off-by: Nikanth Karthikesan <knikanth@...e.de>
> >
> > ---
> >
> > Fix Bug 10504 - losetup possible circular locking
> >
> > Avoid triggering a circular dependency warning by calling fput on the
> > backing file with lo_ctl_mutex held. If the backing file is a device, fput
> > might try to acquire bd_mutex of a that device which triggers a circular
> > dependency warning.
> >
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index edbaac6..5588f67 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -942,11 +942,18 @@ static int loop_clr_fd(struct loop_device *lo, struct
> > block_device *bdev) bd_set_size(bdev, 0);
> >  	mapping_set_gfp_mask(filp->f_mapping, gfp);
> >  	lo->lo_state = Lo_unbound;
> > -	fput(filp);
> >  	/* This is safe: open() is still holding a reference. */
> >  	module_put(THIS_MODULE);
> >  	if (max_part > 0)
> >  		ioctl_by_bdev(bdev, BLKRRPART, 0);
> > +	mutex_unlock(&lo->lo_ctl_mutex);
> > +	/*
> > +	 * Need not hold lo_ctl_mutex to fput backing file.
> > +	 * Calling fput holding lo_ctl_mutex triggers a circular
> > +	 * lock dependency possibility warning as fput can take
> > +	 * bd_mutex which is usually taken before lo_ctl_mutex.
> > +	 */
> > +	fput(filp);
> >  	return 0;
> >  }
> >
> > @@ -1173,7 +1180,10 @@ static int lo_ioctl(struct block_device *bdev,
> > fmode_t mode, err = loop_change_fd(lo, bdev, arg);
> >  		break;
> >  	case LOOP_CLR_FD:
> > +		/* loop_clr_fd would have unlocked lo_ctl_mutex on success */
> >  		err = loop_clr_fd(lo, bdev);
> > +		if (!err)
> > +			goto out_unlocked;
> >  		break;
> >  	case LOOP_SET_STATUS:
> >  		err = loop_set_status_old(lo, (struct loop_info __user *) arg);
> > @@ -1191,6 +1201,8 @@ static int lo_ioctl(struct block_device *bdev,
> > fmode_t mode, err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
> >  	}
> >  	mutex_unlock(&lo->lo_ctl_mutex);
> > +
> > +out_unlocked:
> >  	return err;
> >  }
> 
> 

-- 
Jens Axboe

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