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    SubjectRe: [PATCH 1/11] Add generic helpers for arch IPI function calls
    On Tue, Apr 22 2008, Jens Axboe wrote:
    > On Tue, Apr 22 2008, Avi Kivity wrote:
    > > Jens Axboe wrote:
    > > >This adds kernel/smp.c which contains helpers for IPI function calls. In
    > > >addition to supporting the existing smp_call_function() in a more efficient
    > > >manner, it also adds a more scalable variant called
    > > >smp_call_function_single()
    > > >for calling a given function on a single CPU only.
    > > >
    > > >The core of this is based on the x86-64 patch from Nick Piggin, lots of
    > > >changes since then. "Alan D. Brunelle" <Alan.Brunelle@hp.com> has
    > > >contributed lots of fixes and suggestions as well.
    > > >
    > > >+int smp_call_function_single(int cpu, void (*func) (void *info), void
    > > >*info,
    > > >+ int retry, int wait)
    > > >+{
    > > >+ unsigned long flags;
    > > >+ /* prevent preemption and reschedule on another processor */
    > > >+ int me = get_cpu();
    > > >+ int ret = 0;
    > > >+
    > > >+ /* Can deadlock when called with interrupts disabled */
    > > >+ WARN_ON(wait && irqs_disabled());
    > > >+
    > > >+ if (cpu == me) {
    > > >+ local_irq_save(flags);
    > > >+ func(info);
    > > >+ local_irq_restore(flags);
    > > >+ } else {
    > > >+ struct call_single_data d;
    > > >+ struct call_single_data *data;
    > > >+
    > > >+ if (!wait) {
    > > >+ data = kmalloc(sizeof(*data), GFP_ATOMIC);
    > > >+ if (unlikely(!data)) {
    > > >+ ret = -ENOMEM;
    > > >+ goto out;
    > > >+ }
    > > >+ data->flags = CSD_FLAG_ALLOC;
    > > >+ } else {
    > > >+ data = &d;
    > > >+ data->flags = CSD_FLAG_WAIT;
    > > >+ }
    > > >+
    > > >
    > >
    > > Instead of introducing a rare error case, how about falling back to the
    > > wait case if the allocation fails?
    > >
    > > Of course, if the called function relies on the calling cpu doing
    > > something else, then this fails, but I don't think anybody would do
    > > that? On the other hand, there is at least one use of
    > > smp_call_function_single() with !wait, which doesn't check the error return.
    >
    > Sure, either failling back to waiting, or add a static call_single_data
    > like it exists for smp_call_function(). In reality it'll never happen,
    > so the fallback static structure appeals the most to me.

    We don't need any extra statically allocated data, we can just reuse the
    'csd' element of the existing call_data_fallback. So that is what I did.
    Once all archs are converted, we can now change
    smp_call_function_single() to a void return, as it always succeeds now.

    --
    Jens Axboe



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