Hi Heiko,
On 6/7/07, Heiko Carstens <[email protected]> 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.
Satyam
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