Messages in this thread | | | Date | Tue, 18 Dec 2012 21:23:36 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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On 12/14/2012 11:33 PM, Oleg Nesterov wrote: > On 12/13, Srivatsa S. Bhat wrote: >> >> On 12/13/2012 09:47 PM, Oleg Nesterov wrote: >>> On 12/13, Srivatsa S. Bhat wrote: >>>> >>>> On 12/13/2012 12:42 AM, Srivatsa S. Bhat wrote: >>>>> >>>>> Even I don't spot anything wrong with it. But I'll give it some more >>>>> thought.. >>>> >>>> Since an interrupt handler can also run get_online_cpus_atomic(), we >>>> cannot use the __this_cpu_* versions for modifying reader_percpu_refcnt, >>>> right? >>> >>> Hmm. I thought that __this_cpu_* must be safe under preempt_disable(). >>> IOW, I thought that, say, this_cpu_inc() is "equal" to preempt_disable + >>> __this_cpu_inc() correctness-wise. >>> >>> And. I thought that this_cpu_inc() is safe wrt interrupt, like local_t. >>> >>> But when I try to read the comments percpu.h, I am starting to think that >>> even this_cpu_inc() is not safe if irq handler can do the same? >>> >> >> The comment seems to say that its not safe wrt interrupts. But looking at >> the code in include/linux/percpu.h, IIUC, that is true only about >> this_cpu_read() because it only disables preemption. >> >> However, this_cpu_inc() looks safe wrt interrupts because it wraps the >> increment within raw_local_irqsave()/restore(). > > You mean _this_cpu_generic_to_op() I guess. So yes, I think you are right, > this_cpu_* should be irq-safe, but __this_cpu_* is not. >
Yes.
> Thanks. > > At least on x86 there is no difference between this_ and __this_, both do > percpu_add_op() without local_irq_disable/enable. But it seems that most > of architectures use generic code. >
So now that we can't avoid disabling and enabling interrupts, I was wondering if we could exploit this to avoid the smp_mb()..
Maybe this is a stupid question, but I'll shoot it anyway... Does local_irq_disable()/enable provide any ordering guarantees by any chance? I think the answer is no, but if it is yes, I guess we can do as shown below to ensure that STORE(reader_percpu_refcnt) happens before LOAD(writer_signal).
void get_online_cpus_atomic(void) { unsigned long flags;
preempt_disable();
//only for writer local_irq_save(flags); __this_cpu_add(reader_percpu_refcnt, XXXX); local_irq_restore(flags);
//no need of an explicit smp_mb()
if (__this_cpu_read(reader_percpu_refcnt) & MASK) { this_cpu_inc(reader_percpu_refcnt); } else if (writer_active()) { ... }
this_cpu_dec(reader_percpu_refcnt, XXXX);
}
I tried thinking about other ways to avoid that smp_mb() in the reader, but was unsuccessful. So if the above assumption is wrong, I guess we'll just have to go with the version that uses synchronize_sched() at the writer-side.
Regards, Srivatsa S. Bhat
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