On Mon, 30 Jan 2023 at 21:51, <andrey.konovalov@xxxxxxxxx> wrote:
>
> From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>
> Accesses to slab_index are protected by slab_lock everywhere except
> in a sanity check in stack_depot_fetch. The read access there can race
> with the write access in depot_alloc_stack.
>
> Use WRITE/READ_ONCE() to annotate the racy accesses.
>
> As the sanity check is only used to print a warning in case of a
> violation of the stack depot interface usage, it does not make a lot
> of sense to use proper synchronization.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> ---
> lib/stackdepot.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index f291ad6a4e72..cc2fe8563af4 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -269,8 +269,11 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> return NULL;
> }
>
> - /* Move on to the next slab. */
> - slab_index++;
> + /*
> + * Move on to the next slab.
> + * WRITE_ONCE annotates a race with stack_depot_fetch.
"Pairs with potential concurrent read in stack_depot_fetch()." would be clearer.
I wouldn't say WRITE_ONCE annotates a race (race = involves 2+
accesses, but here's just 1), it just marks this access here which
itself is paired with the potential racing read in the other function.
> + */
> + WRITE_ONCE(slab_index, slab_index + 1);
> slab_offset = 0;
> /*
> * smp_store_release() here pairs with smp_load_acquire() in
> @@ -492,6 +495,8 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
> unsigned long **entries)
> {
> union handle_parts parts = { .handle = handle };
> + /* READ_ONCE annotates a race with depot_alloc_stack. */
> + int slab_index_cached = READ_ONCE(slab_index);
> void *slab;
> size_t offset = parts.offset << DEPOT_STACK_ALIGN;
> struct stack_record *stack;
> @@ -500,9 +505,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
> if (!handle)
> return 0;
>
> - if (parts.slab_index > slab_index) {
> + if (parts.slab_index > slab_index_cached) {
> WARN(1, "slab index %d out of bounds (%d) for stack id %08x\n",
> - parts.slab_index, slab_index, handle);
> + parts.slab_index, slab_index_cached, handle);
> return 0;
> }
> slab = stack_slabs[parts.slab_index];
> --
> 2.25.1
>
