[PATCH] hazptr: handle NULL address in hazptr_detach

[Mathieu Desnoyers]

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.

Fixes: 6357ec235c59 ("hazptrtorture: Fix hazptr ownership issue")
Reported-by: kernel test robot <yi1.lai@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202608130915.62b53936-lkp@xxxxxxxxx
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Boqun Feng <boqun@xxxxxxxxxx>
Cc: Bradley Morgan <brads@xxxxxxxxxxxxxx>
Cc: <rcu@xxxxxxxxxxxxxxx>
Cc: <lkmm@xxxxxxxxxxxxxxx>
---
 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);
-- 
2.43.0