On 8 September 2026 16:22:14 BST, Mathieu Desnoyers
<[email protected]> wrote:
>When hazptr_acquire loads a NULL pointer, it sets:
>
>- slot_item->slot.addr = NULL,
>- slot_item->ctx.ctx = ctx
>- ctx->slot = slot
>
>And it returns NULL.
>
>Then hazptr_detach is called on this ctx, it will act on the ctx as if
>needed to be promoted to backup slot, even though it has a NULL addr.
>
>Looking at what hazptr_note_context_switch() does before promoting
>to backup slot, it checks for a NULL slot->addr, which is exactly
>what is missing from hazptr_detach.
>
>With this in place there would be no need to explicitly check the
>hazptr_acquire() return value before calling hazptr_detach().
>
>hazptr_release() has a early return check for NULL addr as well, so it
>makes sense that detach does an early return (no-op) similarly.
>
You shall kill me for this!!
Could you perhaps do a splat in ze commit description pls?
>Fixes: 6357ec235c59 ("hazptrtorture: Fix hazptr ownership issue")
>Reported-by: kernel test robot <[email protected]>
>Closes: https://lore.kernel.org/oe-lkp/[email protected]
>Signed-off-by: Mathieu Desnoyers <[email protected]>
>Reviewed-by: Bradley Morgan <[email protected]>
>Cc: Paul E. McKenney <[email protected]>
>Cc: Boqun Feng <[email protected]>
>Cc: Bradley Morgan <[email protected]>
>Cc: <[email protected]>
>Cc: <[email protected]>
>---
> include/linux/hazptr.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h
>index 43122c5673bd..d1670121947a 100644
>--- a/include/linux/hazptr.h
>+++ b/include/linux/hazptr.h
>@@ -160,10 +160,12 @@ void hazptr_detach(struct hazptr_ctx *ctx)
> struct hazptr_slot *slot;
>
> guard(preempt)();
>+ slot = ctx->slot;
>+ if (!slot->addr)
>+ return;
> #ifdef CONFIG_HAZPTR_DEBUG
> ctx->detach_task = ctx->detach_cpu = true;
> #endif
>- slot = ctx->slot;
> if (unlikely(hazptr_slot_is_backup(ctx, slot)))
> return;
> hazptr_promote_to_backup_slot(ctx, slot);
>
--- Thanks!
https://lore.kernel.org/all/[email protected]/
