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

[Dave Airlie]

On Thu, Dec 4, 2008 at 5:49 AM, Greg KH <gregkh@xxxxxxx> 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@xxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxx>
> Cc: Clement Calmels <cboulte@xxxxxxxxx>
> Cc: Nadia Derbey <Nadia.Derbey@xxxxxxxx>
> Cc: Pierre Peiffer <peifferp@xxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
>
> ---
>  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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