Re: [patch 021/104] lib/idr.c: fix rcu related race with idr_find

Dave Airlie
2008-12-09T21:41:22Z

On Thu, Dec 4, 2008 at 5:49 AM, Greg KH <gregkh@suse.de> wrote:
> 2.6.27-stable review patch. If anyone has any objections, please let us know.
>
Revert.

This caused problems in the F10 kernel with idr, the drm device alloc
went all wierd,
it might be a drm bug but changing this code triggers it and so it
isn't really "stable"

Dave.

> ------------------
> From: Manfred Spraul <manfred@colorfullife.com>
>
> commit 6ff2d39b91aec3dcae951afa982059e3dd9b49dc upstream.
>
> 2nd part of the fixes needed for
> http://bugzilla.kernel.org/show_bug.cgi?id=11796.
>
> When the idr tree is either grown or shrunk, then the update to the number
> of layers and the top pointer were not atomic. This race caused crashes.
>
> The attached patch fixes that by replicating the layers counter in each
> layer, thus idr_find doesn't need idp->layers anymore.
>
> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
> Cc: Clement Calmels <cboulte@gmail.com>
> Cc: Nadia Derbey <Nadia.Derbey@bull.net>
> Cc: Pierre Peiffer <peifferp@gmail.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
>
> ---
> include/linux/idr.h | 3 ++-
> lib/idr.c | 14 ++++++++++++--
> 2 files changed, 14 insertions(+), 3 deletions(-)
>
> --- a/include/linux/idr.h
> +++ b/include/linux/idr.h
> @@ -52,13 +52,14 @@ struct idr_layer {
> unsigned long bitmap; /* A zero bit means "space here" */
> struct idr_layer *ary[1<<IDR_BITS];
> int count; /* When zero, we can release it */
> + int layer; /* distance from leaf */
> struct rcu_head rcu_head;
> };
>
> struct idr {
> struct idr_layer *top;
> struct idr_layer *id_free;
> - int layers;
> + int layers; /* only valid without concurrent changes */
> int id_free_cnt;
> spinlock_t lock;
> };
> --- a/lib/idr.c
> +++ b/lib/idr.c
> @@ -185,6 +185,7 @@ static int sub_alloc(struct idr *idp, in
> new = get_from_free_list(idp);
> if (!new)
> return -1;
> + new->layer = l-1;
> rcu_assign_pointer(p->ary[m], new);
> p->count++;
> }
> @@ -210,6 +211,7 @@ build_up:
> if (unlikely(!p)) {
> if (!(p = get_from_free_list(idp)))
> return -1;
> + p->layer = 0;
> layers = 1;
> }
> /*
> @@ -237,6 +239,7 @@ build_up:
> }
> new->ary[0] = p;
> new->count = 1;
> + new->layer = layers-1;
> if (p->bitmap == IDR_FULL)
> __set_bit(0, &new->bitmap);
> p = new;
> @@ -493,17 +496,21 @@ void *idr_find(struct idr *idp, int id)
> int n;
> struct idr_layer *p;
>
> - n = idp->layers * IDR_BITS;
> p = rcu_dereference(idp->top);
> + if (!p)
> + return NULL;
> + n = (p->layer+1) * IDR_BITS;
>
> /* Mask off upper bits we don't use for the search. */
> id &= MAX_ID_MASK;
>
> if (id >= (1 << n))
> return NULL;
> + BUG_ON(n == 0);
>
> while (n > 0 && p) {
> n -= IDR_BITS;
> + BUG_ON(n != p->layer*IDR_BITS);
> p = rcu_dereference(p->ary[(id >> n) & IDR_MASK]);
> }
> return((void *)p);
> @@ -582,8 +589,11 @@ void *idr_replace(struct idr *idp, void
> int n;
> struct idr_layer *p, *old_p;
>
> - n = idp->layers * IDR_BITS;
> p = idp->top;
> + if (!p)
> + return ERR_PTR(-EINVAL);
> +
> + n = (p->layer+1) * IDR_BITS;
>
> id &= MAX_ID_MASK;
>
>
> --
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