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    SubjectRe: [PATCH] md: Avoid a deadlock when removing a device from an md array via sysfs.
    On Mon, 2 Apr 2007 17:44:17 +1000 NeilBrown <neilb@suse.de> wrote:

    > (This patch should go in 2.6.21 as it fixes a recent regression - NB)
    >
    > A device can be removed from an md array via e.g.
    > echo remove > /sys/block/md3/md/dev-sde/state
    >
    > This will try to remove the 'dev-sde' subtree which will deadlock
    > since
    > commit e7b0d26a86943370c04d6833c6edba2a72a6e240
    >
    > With this patch we run the kobject_del via schedule_work so as to
    > avoid the deadlock.
    >
    > Cc: Alan Stern <stern@rowland.harvard.edu>
    > Signed-off-by: Neil Brown <neilb@suse.de>
    >
    > ### Diffstat output
    > ./drivers/md/md.c | 13 ++++++++++++-
    > ./include/linux/raid/md_k.h | 1 +
    > 2 files changed, 13 insertions(+), 1 deletion(-)
    >
    > diff .prev/drivers/md/md.c ./drivers/md/md.c
    > --- .prev/drivers/md/md.c 2007-04-02 17:43:03.000000000 +1000
    > +++ ./drivers/md/md.c 2007-04-02 17:38:46.000000000 +1000
    > @@ -1389,6 +1389,12 @@ static int bind_rdev_to_array(mdk_rdev_t
    > return err;
    > }
    >
    > +static void delayed_delete(struct work_struct *ws)
    > +{
    > + mdk_rdev_t *rdev = container_of(ws, mdk_rdev_t, del_work);
    > + kobject_del(&rdev->kobj);
    > +}
    > +
    > static void unbind_rdev_from_array(mdk_rdev_t * rdev)
    > {
    > char b[BDEVNAME_SIZE];
    > @@ -1401,7 +1407,12 @@ static void unbind_rdev_from_array(mdk_r
    > printk(KERN_INFO "md: unbind<%s>\n", bdevname(rdev->bdev,b));
    > rdev->mddev = NULL;
    > sysfs_remove_link(&rdev->kobj, "block");
    > - kobject_del(&rdev->kobj);
    > +
    > + /* We need to delay this, otherwise we can deadlock when
    > + * writing to 'remove' to "dev/state"
    > + */
    > + INIT_WORK(&rdev->del_work, delayed_delete);
    > + schedule_work(&rdev->del_work);
    > }
    >
    > /*
    >
    > diff .prev/include/linux/raid/md_k.h ./include/linux/raid/md_k.h
    > --- .prev/include/linux/raid/md_k.h 2007-04-02 17:43:03.000000000 +1000
    > +++ ./include/linux/raid/md_k.h 2007-04-02 17:36:32.000000000 +1000
    > @@ -104,6 +104,7 @@ struct mdk_rdev_s
    > * for reporting to userspace and storing
    > * in superblock.
    > */
    > + struct work_struct del_work; /* used for delayed sysfs removal */
    > };
    >

    What guarantees that *rdev is still valid when delayed_delete() runs?

    And what guarantees that the md module hasn't been rmmodded when
    delayed_delete() tries to run?

    -
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