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    SubjectRe: [PATCH] Fix Bug 10504 - losetup possible circular locking
    Date
    Hi Jens

    Did you get to look at this? Can you ACK/NACK this one?

    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;
    > }




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