Re: [ANNOUNCE] v5.14-rc5-rt8

[Mike Galbraith]

On Sat, 2021-08-14 at 21:08 +0200, Vlastimil Babka wrote:
>
> Aha! That's helpful. Hopefully it's just a small issue where we
> opportunistically test flags on a page that's protected by the local
> lock we didn't take yet, and I didn't realize there's the VM_BUG_ON
> which can trigger if our page went away (which we would have realized
> after taking the lock).

Speaking of optimistic peeking perhaps going badly, why is the below
not true?  There's protection against ->partial going disappearing
during a preemption... but can't it just as easily appear, so where is
that protection?

If the other side of that window is safe, it could use a comment so
dummies reading this code don't end up asking mm folks why the heck
they don't just take the darn lock and be done with it instead of tap
dancing all around thething :)

---
 mm/slub.c |   14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2937,17 +2937,16 @@ static void *___slab_alloc(struct kmem_c

 new_slab:

+	/*
+	 * To avoid false negative race with put_cpu_partial() during a
+	 * preemption, we must call slub_percpu_partial() under lock.
+	 */
+	local_lock_irqsave(&s->cpu_slab->lock, flags);
 	if (slub_percpu_partial(c)) {
-		local_lock_irqsave(&s->cpu_slab->lock, flags);
 		if (unlikely(c->page)) {
 			local_unlock_irqrestore(&s->cpu_slab->lock, flags);
 			goto reread_page;
 		}
-		if (unlikely(!slub_percpu_partial(c))) {
-			local_unlock_irqrestore(&s->cpu_slab->lock, flags);
-			/* we were preempted and partial list got empty */
-			goto new_objects;
-		}

 		page = c->page = slub_percpu_partial(c);
 		slub_set_percpu_partial(c, page);
@@ -2955,8 +2954,7 @@ static void *___slab_alloc(struct kmem_c
 		stat(s, CPU_PARTIAL_ALLOC);
 		goto redo;
 	}
-
-new_objects:
+	local_unlock_irqrestore(&s->cpu_slab->lock, flags);

 	freelist = get_partial(s, gfpflags, node, &page);
 	if (freelist)