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    SubjectRe: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems_allowed
    On Thu, Mar 11, 2010 at 01:04:33PM +0800, Miao Xie wrote:
    > on 2010-3-10 3:42, Paul Menage wrote:
    > > On Sat, Mar 6, 2010 at 6:33 PM, Miao Xie <miaox@cn.fujitsu.com> wrote:
    > >>
    > >> Before applying this patch, cpuset updates task->mems_allowed just like
    > >> what you said. But the allocator is still likely to see an empty nodemask.
    > >> This problem have been pointed out by Nick Piggin.
    > >>
    > >> The problem is following:
    > >> The size of nodemask_t is greater than the size of long integer, so loading
    > >> and storing of nodemask_t are not atomic operations. If task->mems_allowed
    > >> don't intersect with new_mask, such as the first word of the mask is empty
    > >> and only the first word of new_mask is not empty. When the allocator
    > >> loads a word of the mask before
    > >>
    > >> current->mems_allowed |= new_mask;
    > >>
    > >> and then loads another word of the mask after
    > >>
    > >> current->mems_allowed = new_mask;
    > >>
    > >> the allocator gets an empty nodemask.
    > >
    > > Couldn't that be solved by having the reader read the nodemask twice
    > > and compare them? In the normal case there's no race, so the second
    > > read is straight from L1 cache and is very cheap. In the unlikely case
    > > of a race, the reader would keep trying until it got two consistent
    > > values in a row.
    >
    > I think this method can't fix the problem because we can guarantee the second
    > read is after the update of mask completes.

    Any problem with using a seqlock?

    The other thing you could do is store a pointer to the nodemask, and
    allocate a new nodemask when changing it, issue a smp_wmb(), and then
    store the new pointer. Read side only needs a smp_read_barrier_depends()



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