On 4/2/24 2:14 AM, Peter Collingbourne wrote:
> Commit 3ee34eabac2a ("lib/stackdepot: fix first entry having a
> 0-handle") changed the meaning of the pool_index field to mean "the
> pool index plus 1". This made the code accessing this field less
> self-documenting, as well as causing debuggers such as drgn to not
> be able to easily remain compatible with both old and new kernels,
> because they typically do that by testing for presence of the new
> field. Because stackdepot is a debugging tool, we should make sure
> that it is debugger friendly. Therefore, give the field a different
> name to improve readability as well as enabling debugger backwards
> compatibility.
>
> Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> Link: https://linux-review.googlesource.com/id/Ib3e70c36c1d230dd0a118dc22649b33e768b9f88
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> Although this technically isn't a bug fix in the kernel,
> I would appreciate if this could be picked up for 6.9
> to avoid temporarily introducing a silent regression in drgn
> (loud regressions are fine).
>
> include/linux/stackdepot.h | 7 +++----
> lib/stackdepot.c | 4 ++--
> 2 files changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index 3c6caa5abc7c..e9ec32fb97d4 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -44,10 +44,9 @@ typedef u32 depot_stack_handle_t;
> union handle_parts {
> depot_stack_handle_t handle;
> struct {
> - /* pool_index is offset by 1 */
> - u32 pool_index : DEPOT_POOL_INDEX_BITS;
> - u32 offset : DEPOT_OFFSET_BITS;
> - u32 extra : STACK_DEPOT_EXTRA_BITS;
> + u32 pool_index_plus_1 : DEPOT_POOL_INDEX_BITS;
> + u32 offset : DEPOT_OFFSET_BITS;
> + u32 extra : STACK_DEPOT_EXTRA_BITS;
> };
> };
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index af6cc19a2003..68c97387aa54 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -330,7 +330,7 @@ static struct stack_record *depot_pop_free_pool(void **prealloc, size_t size)
> stack = current_pool + pool_offset;
>
> /* Pre-initialize handle once. */
> - stack->handle.pool_index = pool_index + 1;
> + stack->handle.pool_index_plus_1 = pool_index + 1;
> stack->handle.offset = pool_offset >> DEPOT_STACK_ALIGN;
> stack->handle.extra = 0;
> INIT_LIST_HEAD(&stack->hash_list);
> @@ -441,7 +441,7 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
> const int pools_num_cached = READ_ONCE(pools_num);
> union handle_parts parts = { .handle = handle };
> void *pool;
> - u32 pool_index = parts.pool_index - 1;
> + u32 pool_index = parts.pool_index_plus_1 - 1;
> size_t offset = parts.offset << DEPOT_STACK_ALIGN;
> struct stack_record *stack;
>
