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SubjectRe: [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@suse.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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