Messages in this thread | | | Date | Mon, 22 Jul 2019 20:51:59 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH v3 1/9] smp: Run functions concurrently in smp_call_function_many() |
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On Mon, 22 Jul 2019, Nadav Amit wrote: > > On Jul 22, 2019, at 11:37 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > > > On Mon, 22 Jul 2019, Peter Zijlstra wrote: > > > >> On Thu, Jul 18, 2019 at 05:58:29PM -0700, Nadav Amit wrote: > >>> +/* > >>> + * Call a function on all processors. May be used during early boot while > >>> + * early_boot_irqs_disabled is set. > >>> + */ > >>> +static inline void on_each_cpu(smp_call_func_t func, void *info, int wait) > >>> +{ > >>> + on_each_cpu_mask(cpu_online_mask, func, info, wait); > >>> +} > >> > >> I'm thinking that one if buggy, nothing protects online mask here. > > > > The current implementation has preemption disabled before touching > > cpu_online_mask which at least protects against a CPU going away as that > > prevents the stomp machine thread from getting on the CPU. But it's not > > protected against a CPU coming online concurrently. > > I still don’t understand. If you called cpu_online_mask() and did not > disable preemption before calling it, you are already (today) not protected > against another CPU coming online. Disabling preemption in on_each_cpu() > will not solve it.
Disabling preemption _cannot_ protect against a CPU coming online. It only can protect against a CPU being offlined.
The current implementation of on_each_cpu() disables preemption _before_ touching cpu_online_mask.
void on_each_cpu(void (*func) (void *info), void *info, int wait) { unsigned long flags;
preempt_disable(); smp_call_function(func, info, wait);
smp_call_function() has another preempt_disable as it can be called separately and it does:
preempt_disable(); smp_call_function_many(cpu_online_mask, func, info, wait);
Your new on_each_cpu() implementation does not. So there is a difference. Whether it matters or not is a different question, but that needs to be explained and documented.
Thanks,
tglx
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