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    SubjectRe: [PATCH -v2] use per cpu data for single cpu ipi calls
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    On Thu, 2009-01-29 at 18:47 +0100, Peter Zijlstra wrote:
    > On Thu, 2009-01-29 at 09:21 -0800, Linus Torvalds wrote:
    > > On Thu, 29 Jan 2009, Steven Rostedt wrote:
    > > >
    > > > The caller must wait till the LOCK bit is cleared before setting
    > > > it. When it is cleared, there is no IPI function using it.
    > > > A spinlock is used to synchronize the setting of the bit between
    > > > callers. Since only one callee can be called at a time, and it
    > > > is the only thing to clear it, the IPI does not need to use
    > > > any locking.
    > >
    > > That spinlock cannot be right. It is provably wrong for so many reasons..
    > >
    > > Think about it. We're talking about a per-CPU lock, which already makes no
    > > sense: we're only locking against our own CPU, and we've already disabled
    > > preemption for totally unrelated reasons.
    > >
    > > And the only way locking can make sense against our own CPU is if we lock
    > > against interrupts - but the lock isn't actually irq-safe, so if you are
    > > trying to lock against interrupts, you are (a) doing it wrong (you should
    > > disable interrupts, not use a spinlock) and (b) causing a deadlock if it
    > > ever happens.
    >
    >
    > > + else {
    > > + data = &per_cpu(csd_data, cpu);
    > > + spin_lock(&per_cpu(csd_data_lock, cpu));
    > > + while (data->flags & CSD_FLAG_LOCK)
    > > + cpu_relax();
    > > + data->flags = CSD_FLAG_LOCK;
    > > + spin_unlock(&per_cpu(csd_data_lock, cpu));
    > > + }
    >
    > I think your argument would hold if he did:
    >
    > data = &__get_cpu_var(csd_data);
    >
    > But now he's actually grabbing the remote cpu's csd, and thus needs
    > atomicy around that remote csd -- which two cpus could contend for.

    So the below should do

    ---
    kernel/smp.c | 6 +-----
    1 files changed, 1 insertions(+), 5 deletions(-)

    diff --git a/kernel/smp.c b/kernel/smp.c
    index 9bce851..9eead6c 100644
    --- a/kernel/smp.c
    +++ b/kernel/smp.c
    @@ -201,8 +201,6 @@ void generic_smp_call_function_single_interrupt(void)
    }

    static DEFINE_PER_CPU(struct call_single_data, csd_data);
    -static DEFINE_PER_CPU(spinlock_t, csd_data_lock) =
    - __SPIN_LOCK_UNLOCKED(csd_lock);

    /*
    * smp_call_function_single - Run a function on a specific CPU
    @@ -259,12 +257,10 @@ int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
    if (data)
    data->flags = CSD_FLAG_ALLOC;
    else {
    - data = &per_cpu(csd_data, cpu);
    - spin_lock(&per_cpu(csd_data_lock, cpu));
    + data = &per_cpu(csd_data, me);
    while (data->flags & CSD_FLAG_LOCK)
    cpu_relax();
    data->flags = CSD_FLAG_LOCK;
    - spin_unlock(&per_cpu(csd_data_lock, cpu));
    }
    } else {
    data = &d;



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