Messages in this thread | | | Date | Thu, 7 Jun 2007 22:24:01 +0530 | From | "Satyam Sharma" <> | Subject | Re: [patch] i386/x86_64: smp_call_function locking inconsistency |
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Hi Heiko,
On 6/7/07, Heiko Carstens <heiko.carstens@de.ibm.com> wrote: > > Replacing the _bh variants and making smp_call_function{_single} > > illegal from all contexts but process is fine for x86_64, as we > > don't really have any driver that needs to use this from softirq > > context in the x86_64 tree. This means it becomes dissimilar to > > s390, but similar to powerpc, mips, alpha, sparc64 semantics. > > I'll prepare and submit a patch for the same, shortly. > > Calling an smp_call_* function from any context but process context is > a bug. We didn't notice this initially when we used smp_call_function > from softirq context... until we deadlocked ;) > So s390 is the same as any other architecture wrt this.
I'll fix the necessary patch for x86_64, in that case.
> > I don't see any CPU hotplug / preemption disabling issues here. > > Note that both smp_call_function() and smp_call_function_single() > > on x86_64 acquire the call_lock spinlock before using cpu_online_map > > via num_online_cpus(). And spin_lock() does preempt_disable() on both > > SMP and !SMP, so we're safe. [ But we're not explicitly disabling > > preemption and depending on spin_lock() instead, so a comment would > > be in order? ] > > Calling smp_call_function_single() with preemption enabled is pointless. > You might be scheduled on the cpu you want to send an IPI to and get > -EBUSY as return... If cpu hotplug is enabled the target cpu might even > be gone when smp_call_function_single() gets executed.
Exactly. I was only mentioning that the smp_call_function* of x86{_64} were safe anyway (but the smp_processor_id() that would've preceded it need not have been, of course).
> Avi Kivity has already a patch which introduces an on_cpu() function which > looks quite like on_each_cpu(). That way you don't have to open code this > stuff over and over again: > > preempt_disable(); > if (cpu == smp_processor_id()) > func(); > else > smp_call_function_single(...); > preempt_enable(); > > There are already quite a few of these around.
Indeed -- this was doubly problematic because the un-safeness was because of smp_processor_id() as well as the (eventual) access of cpu_online_map (via smp_call_function() -> num_online_cpus()) ... thanks for letting me know about this.
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