Messages in this thread | | | Date | Sat, 6 Jul 2013 14:03:17 +0800 | From | Wang YanQing <> | Subject | Re: [PATCH 1/3] smp/ipi: Remove redundant cfd->cpumask_ipi mask |
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On Sat, Jul 06, 2013 at 10:59:39AM +0530, Preeti U Murthy wrote: > Hi Wang, > > On 07/06/2013 08:43 AM, Wang YanQing wrote: > > On Fri, Jul 05, 2013 at 09:57:01PM +0530, Preeti U Murthy wrote: > >> cfd->cpumask_ipi is used only in smp_call_function_many().The existing > >> comment around it says that this additional mask is used because > >> cfd->cpumask can get overwritten. > >> > >> There is no reason why the cfd->cpumask can be overwritten, since this > >> is a per_cpu mask; nobody can change it but us and we are > >> called with preemption disabled. > > > > The ChangeLog for f44310b98ddb7f0d06550d73ed67df5865e3eda5 > > which import cfd->cpumask_ipi saied the reason why we need > > it: > > > > " As explained by Linus as well: > > > > | > > | Once we've done the "list_add_rcu()" to add it to the > > | queue, we can have (another) IPI to the target CPU that can > > | now see it and clear the mask. > > | > > | So by the time we get to actually send the IPI, the mask might > > | have been cleared by another IPI. > > I am unable to understand where the cfd->cpumask of the source cpu is > getting cleared. Surely not by itself, since it is preempt disabled. > Also why should it get cleared?
Assume we have three CPUs: A,B,C
A call smp_call_function_many to notify C do something, and current it execute on finished below codes:
"for_each_cpu(cpu, cfd->cpumask) { struct call_single_data *csd = per_cpu_ptr(cfd->csd, cpu); struct call_single_queue *dst = &per_cpu(call_single_queue, cpu); unsigned long flags;
csd_lock(csd); csd->func = func; csd->info = info;
raw_spin_lock_irqsave(&dst->lock, flags); list_add_tail(&csd->list, &dst->list); raw_spin_unlock_irqrestore(&dst->lock, flags); } " You see "list_add_tail(&csd->list, &dst->list);", it pass the address of csd, and A stop before call arch_send_call_function_ipi_mask due interrupt.
At this time B send ipi to C also, then C will see the csd passed by A, then C will clear itself in the cfd->cpumask.
Thanks.
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