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    SubjectRe: [PATCH 21/77] block: fix synchronization and limit check in blk_alloc_devt()
    On Wed,  6 Feb 2013 11:39:53 -0800
    Tejun Heo <tj@kernel.org> wrote:

    > idr allocation in blk_alloc_devt() wasn't synchronized against lookup
    > and removal, and its limit check was off by one - 1 << MINORBITS is
    > the number of minors allowed, not the maximum allowed minor.
    >
    > Add locking and rename MAX_EXT_DEVT to NR_EXT_DEVT and fix limit
    > checking.
    >
    > ...
    >
    > --- a/block/genhd.c
    > +++ b/block/genhd.c
    > @@ -26,7 +26,7 @@ static DEFINE_MUTEX(block_class_lock);
    > struct kobject *block_depr;
    >
    > /* for extended dynamic devt allocation, currently only one major is used */
    > -#define MAX_EXT_DEVT (1 << MINORBITS)
    > +#define NR_EXT_DEVT (1 << MINORBITS)
    >
    > /* For extended devt allocation. ext_devt_mutex prevents look up
    > * results from going away underneath its user.
    > @@ -423,17 +423,18 @@ int blk_alloc_devt(struct hd_struct *part, dev_t *devt)
    > do {
    > if (!idr_pre_get(&ext_devt_idr, GFP_KERNEL))
    > return -ENOMEM;
    > + mutex_lock(&ext_devt_mutex);
    > rc = idr_get_new(&ext_devt_idr, part, &idx);
    > + if (!rc && idx >= NR_EXT_DEVT) {
    > + idr_remove(&ext_devt_idr, idx);
    > + rc = -EBUSY;
    > + }
    > + mutex_unlock(&ext_devt_mutex);
    > } while (rc == -EAGAIN);
    >
    > if (rc)
    > return rc;
    >
    > - if (idx > MAX_EXT_DEVT) {
    > - idr_remove(&ext_devt_idr, idx);
    > - return -EBUSY;
    > - }
    > -
    > *devt = MKDEV(BLOCK_EXT_MAJOR, blk_mangle_minor(idx));
    > return 0;
    > }

    This gets all tangled up with
    http://ozlabs.org/~akpm/mmotm/broken-out/block-fix-ext_devt_idr-handling.patch,
    which appears to solve the same issue.

    What to do?



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