Messages in this thread |  | | Subject | Re: [RFC][PATCH] slub: Check for page NULL before doing the node_match check | From | Steven Rostedt <> | Date | Thu, 17 Jan 2013 17:46:03 -0500 |
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On Thu, 2013-01-17 at 21:51 +0000, Christoph Lameter wrote:
> This is dealing with the same cpu being interrupted. Some of these > segments are in interrupt disable sections so they are not affected.
Except that we are not always on the same CPU. Now I'm looking at mainline (non modified by -rt):
From slab_alloc_node():
/* * Must read kmem_cache cpu data via this cpu ptr. Preemption is * enabled. We may switch back and forth between cpus while * reading from one cpu area. That does not matter as long * as we end up on the original cpu again when doing the cmpxchg. */ c = __this_cpu_ptr(s->cpu_slab);
/* * The transaction ids are globally unique per cpu and per operation on * a per cpu queue. Thus they can be guarantee that the cmpxchg_double * occurs on the right processor and that there was no operation on the * linked list in between. */ tid = c->tid; barrier();
object = c->freelist; page = c->page; if (unlikely(!object || !node_match(page, node))) object = __slab_alloc(s, gfpflags, node, addr, c);
Where we hit the bug on -rt, and can most certainly do it on mainline.
This code does not disable preemption (the comment even states that). So if we switch CPUs after reading __this_cpu_ptr(), we are still accessing the 'c' pointer of the CPU we left. Hence, there's nothing protecting c->page being NULL when c->freelist is not NULL.
-- Steve
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