Messages in this thread | | | Subject | Re: [PATCH -v2] use per cpu data for single cpu ipi calls | From | Peter Zijlstra <> | Date | Thu, 29 Jan 2009 18:47:02 +0100 |
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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.
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