Re: [ANNOUNCE] v5.14-rc5-rt8
From: Mike Galbraith <hidden>
Date: 2021-08-15 04:17:31
Also in:
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Possibly related (same subject, not in this thread)
- 2021-08-15 · Re: [ANNOUNCE] v5.14-rc5-rt8 · Mike Galbraith <hidden>
- 2021-08-14 · Re: [ANNOUNCE] v5.14-rc5-rt8 · Vlastimil Babka <hidden>
- 2021-08-14 · Re: [ANNOUNCE] v5.14-rc5-rt8 · Mike Galbraith <hidden>
- 2021-08-13 · Re: [ANNOUNCE] v5.14-rc5-rt8 · Mike Galbraith <hidden>
- 2021-08-12 · Re: [ANNOUNCE] v5.14-rc5-rt8 · Clark Williams <hidden>
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)