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