Nick Piggin <npiggin@xxxxxxxxx> writes:
> Hi,
>
> While hunting down a bug in NFS's AIO, I believe I found this
> buggy code...
>
> fs: aio fix rcu ioctx lookup
>
> aio-dio-invalidate-failure GPFs in aio_put_req from io_submit.
>
> lookup_ioctx doesn't implement the rcu lookup pattern properly.
> rcu_read_lock does not prevent refcount going to zero, so we
> might take a refcount on a zero count ioctx.
So, does this patch fix the problem? You didn't actually say....
> Signed-off-by: Nick Piggin <npiggin@xxxxxxxxx>
>
> Index: linux-2.6/fs/aio.c
> ===================================================================
> --- linux-2.6.orig/fs/aio.c 2011-01-14 00:29:00.000000000 +1100
> +++ linux-2.6/fs/aio.c 2011-01-14 11:31:47.000000000 +1100
> @@ -239,15 +239,23 @@ static void __put_ioctx(struct kioctx *c
> call_rcu(&ctx->rcu_head, ctx_rcu_free);
> }
>
> -#define get_ioctx(kioctx) do { \
> - BUG_ON(atomic_read(&(kioctx)->users) <= 0); \
> - atomic_inc(&(kioctx)->users); \
> -} while (0)
> -#define put_ioctx(kioctx) do { \
> - BUG_ON(atomic_read(&(kioctx)->users) <= 0); \
> - if (unlikely(atomic_dec_and_test(&(kioctx)->users))) \
> - __put_ioctx(kioctx); \
> -} while (0)
> +static inline void get_ioctx(struct kioctx *kioctx)
> +{
> + BUG_ON(atomic_read(&kioctx->users) <= 0);
> + atomic_inc(&kioctx->users);
> +}
> +
> +static inline int try_get_ioctx(struct kioctx *kioctx)
> +{
> + return atomic_inc_not_zero(&kioctx->users);
> +}
> +
> +static inline void put_ioctx(struct kioctx *kioctx)
> +{
> + BUG_ON(atomic_read(&kioctx->users) <= 0);
> + if (unlikely(atomic_dec_and_test(&kioctx->users)))
> + __put_ioctx(kioctx);
> +}
Why did you switch from macros? Personal preference? Can you at least
mention it in the changelog?
>
> /* ioctx_alloc
> * Allocates and initializes an ioctx. Returns an ERR_PTR if it failed.
> @@ -601,8 +609,7 @@ static struct kioctx *lookup_ioctx(unsig
> rcu_read_lock();
>
> hlist_for_each_entry_rcu(ctx, n, &mm->ioctx_list, list) {
> - if (ctx->user_id == ctx_id && !ctx->dead) {
> - get_ioctx(ctx);
> + if (ctx->user_id == ctx_id && !ctx->dead && try_get_ioctx(ctx)){
> ret = ctx;
> break;
> }
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