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    SubjectRe: [PATCH v2 3/3] mm/slub: Use percpu partial free counter
    On Wed, Mar 03, 2021 at 08:55:48PM +0100, Christoph Lameter wrote:
    > On Wed, 3 Mar 2021, Matthew Wilcox wrote:
    >
    > > > Can this be allocated in an interrupt context?
    > > >
    > > > And I am not sure how local_t relates to that? Percpu counters can be used
    > > > in an interrupt context without the overhead of the address calculations
    > > > that are required by a local_t.
    > >
    > > As I understand the patch, this counts the number of partially free slabs.
    > > So if we start to free an object from a completely full slab in process
    > > context, as "load x, add one to x, store x" and take an interrupt
    > > between loading x and adding one to x, that interrupt handler might
    > > free a different object from another completely full slab. that would
    > > also load the same x, add one to it and store x, but then the process
    > > context would add one to the old x, overwriting the updated value from
    > > interrupt context.
    >
    > this_cpu operations are "atomic" vs. preemption but on some platforms not
    > vs interrupts. That could be an issue in kmem_cache_free(). This would
    > need a modification to the relevant this_cpu ops so that interrupts are
    > disabled on those platforms.

    Hmmmm ... re-reading the documentation, it says that this_cpu_x is
    atomic against interrupts:

    These operations can be used without worrying about
    preemption and interrupts::
    [...]
    this_cpu_add(pcp, val)
    this_cpu_inc(pcp)
    ...

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