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Date: Tue, 02 Jul 2013 16:47:24 +0800 From: Michael Wang <wangyun@...ux.vnet.ibm.com> To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> CC: tglx@...utronix.de, peterz@...radead.org, tj@...nel.org, oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au, mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org, walken@...gle.com, vincent.guittot@...aro.org, laijs@...fujitsu.com, David.Laight@...lab.com, rostedt@...dmis.org, xiaoguangrong@...ux.vnet.ibm.com, sbw@....edu, fweisbec@...il.com, zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org, linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Wang YanQing <udknight@...il.com>, Shaohua Li <shli@...ionio.com>, Jan Beulich <jbeulich@...e.com>, liguang <lig.fnst@...fujitsu.com> Subject: Re: [PATCH v3 10/45] smp: Use get/put_online_cpus_atomic() to prevent CPU offline On 07/02/2013 04:25 PM, Srivatsa S. Bhat wrote: > Hi Michael, > > On 07/02/2013 11:02 AM, Michael Wang wrote: >> Hi, Srivatsa >> >> On 06/28/2013 03:54 AM, Srivatsa S. Bhat wrote: >> [snip] >>> @@ -625,8 +632,9 @@ EXPORT_SYMBOL(on_each_cpu_mask); >>> * The function might sleep if the GFP flags indicates a non >>> * atomic allocation is allowed. >>> * >>> - * Preemption is disabled to protect against CPUs going offline but not online. >>> - * CPUs going online during the call will not be seen or sent an IPI. >>> + * We use get/put_online_cpus_atomic() to protect against CPUs going >>> + * offline but not online. CPUs going online during the call will >>> + * not be seen or sent an IPI. >> >> I was a little confused about this comment, if the offline-cpu still >> have chances to become online, then there is chances that we will pick >> it from for_each_online_cpu(), isn't it? Did I miss some point? >> > > Whether or not the newly onlined CPU is observed in our for_each_online_cpu() > loop, is dependent on timing. On top of that, there are 2 paths in the code: > one which uses a temporary cpumask and the other which doesn't. In the former > case, if a CPU comes online _after_ we populate the temporary cpumask, then > we won't send an IPI to that cpu, since the temporary cpumask doesn't contain > that CPU. Whereas, if we observe the newly onlined CPU in the for_each_online_cpu() > loop itself (either in the former or the latter case), then yes, we will send > the IPI to that CPU. So it is not 'during the call' but 'during the call of on_each_cpu_mask()', correct? The comment position seems like it declaim that during the call of this func, online-cpu won't be seem and send IPI... Regards, Michael Wang > > Regards, > Srivatsa S. Bhat > >> >>> * >>> * You must not call this function with disabled interrupts or >>> * from a hardware interrupt handler or from a bottom half handler. >>> @@ -641,26 +649,26 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info), >>> might_sleep_if(gfp_flags & __GFP_WAIT); >>> >>> if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) { >>> - preempt_disable(); >>> + get_online_cpus_atomic(); >>> for_each_online_cpu(cpu) >>> if (cond_func(cpu, info)) >>> cpumask_set_cpu(cpu, cpus); >>> on_each_cpu_mask(cpus, func, info, wait); >>> - preempt_enable(); >>> + put_online_cpus_atomic(); >>> free_cpumask_var(cpus); >>> } else { >>> /* >>> * No free cpumask, bother. No matter, we'll >>> * just have to IPI them one by one. >>> */ >>> - preempt_disable(); >>> + get_online_cpus_atomic(); >>> for_each_online_cpu(cpu) >>> if (cond_func(cpu, info)) { >>> ret = smp_call_function_single(cpu, func, >>> info, wait); >>> WARN_ON_ONCE(!ret); >>> } >>> - preempt_enable(); >>> + put_online_cpus_atomic(); >>> } >>> } >>> EXPORT_SYMBOL(on_each_cpu_cond); >>> >> > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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