RE: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing:convert bts-tracer mutex to a spinlock

[Metzger, Markus T]

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@xxxxxxx]
>Sent: Wednesday, February 25, 2009 12:46 PM
>To: Metzger, Markus T


>> I suggested to use get/put_online_cpus() instead of the lock.
>>
>> The hotplug code waits until the cpu_hotplug.refcount is zero
>> and it holds the cpu_hotplug.lock during hotplug operations
>> (see cpu_hotplug_begin/done()).
>>
>> In turn, get_online_cpus() needs to grab the cpu_hotplug.lock
>> to increment the cpu_hotplug.refcount.
>>
>> Thus, we will use the cpu_hotplug.lock instead of our own lock.
>
>... which, if you use it in the exact same spots as now still
>does a potential sleep with irqs disabled => bad.

We don't need to get_online_cpus() in the hotplug handler;
actually, we must not.


>We might be able to not take the hotplug lock in the affected
>codepath, but we should really not expand on the use of that
>lock and should make this code self-sufficient.

You're right. An own spinlock is definitely safer and much
easier to understand.


thanks and regards,
markus.

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