Re: [RFC][PATCH] slub: Check for page NULL before doing thenode_match check

[Steven Rostedt]

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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