linux-kernel - [Slub allocator] There are chances that kmem_cache_cpu->freelist is lost if the process is rescheduled to a differenet cpu before the local irq is disabled when calling __slab

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Date:	Sun, 11 Dec 2011 17:52:47 +0800
From:	zhihua che <zhihua.che@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: [Slub allocator] There are chances that kmem_cache_cpu->freelist is
 lost if the process is rescheduled to a differenet cpu before the local irq
 is disabled when calling __slab_alloc()

Hi, everyone
        I'm reading the linux kernel and now focusing on memory
management. Image a case like below, a process on cpu0 is calling
slab_alloc for a free obj, and finds that the c0->freelist is null,
where c0 is the pointer to a kmem_cache_cpu object corresponding to
the current cpu0. Then the process resorts to the __slab_alloc and
HERE, the process MAY BE rescheduled to a different cpu1 before the
local irq is disabled. Then, a new kmem_cache_cpu pointer c1 is
retrieved. I think there are chances that with respect to this
different kmem_cache_cpu pointer c1, c1->freelist is NOT null and
could be lost if the  following code is executed in __slab_alloc:

         load_freelist:
                 VM_BUG_ON(!page->frozen);
                 c->freelist = get_free_pointer(s, object);
/* THIS ASSIGNMENT */
                 c->tid = next_tid(c->tid);
                 local_irq_restore(flags);
                 return object;

         I think the assignment above may cause the loss of the
freelist pointed by c->freelist, because c now  points to a different
kmem_cache_cpu object due the reschedule and this freelist may be NOT
null as I imaged earlier.
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