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    SubjectRe: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub
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    On Wed, 2010-11-24 at 10:14 -0600, Christoph Lameter wrote:
    > On Wed, 24 Nov 2010, Peter Zijlstra wrote:
    >
    > > This thing still relies on disabling IRQs in the slow path, which means
    > > its still going to be a lot of work to make it work on -rt.
    >
    > The disabling of irqs is because slab operations are used from interrupt
    > context. If we can avoid slab operations from interrupt contexts then we
    > can drop the interrupt disable in the slab allocators.

    That's not so much the point, there's per-cpu assumptions due to that.
    Not everything is under a proper lock, see for example this bit:

    new = new_slab(s, gfpflags, node);

    if (gfpflags & __GFP_WAIT)
    local_irq_disable();

    if (new) {
    c = __this_cpu_ptr(s->cpu_slab);
    stat(s, ALLOC_SLAB);
    if (c->page)
    flush_slab(s, c);
    slab_lock(new);
    __SetPageSlubFrozen(new);
    c->page = new;
    goto load_freelist;
    }

    There we have the __this_cpu_ptr, c->page deref and flush_slab()->stat()
    call all before we take a lock.




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