On Thu, 11 Jul 2013, Srivatsa S. Bhat wrote:
> Oops! You are right. Hmm, this looks quite difficult to get right :(
> There are multiple challenges here:
>
> 1. The sysfs files must not be removed during cpu_down, and not initialized
>
> during cpu_up. That would help us preserve the file permissions.
> 2. But we should ensure that we really do the cpufreq-core parts of the cpu
> initialization during cpu_up. If we fail to free some of the
> data-structures
> during cpu_down, the cpu_up callback will think that a full-init is not
> required and not do its job. That will make cpufreq behave erratically
> after
> suspend/resume and take us back to square one.
>
> 3. A full re-init in the cpu_up callback also involves memory allocations.
> So if we don't release the memory in the cpu_down callback, we'll end up
> in a memory leak.
>
> I tried to address all these in this patch, but you found yet another serious
> loop-hole. I guess I'm out of ideas now... if anybody has any thoughts on how
> to get this right, then I'm all ears. Else, we'll just revert the original
> commit like Rafael suggested and leave it upto userspace to save and restore
> the permissions across suspend/resume if it wants ;-(
Asking as a naive outsider who is completely unfamiliar with the code,
why are any of these things at all troublesome?
Can't cpu_up tell the difference between activating a brand-new
CPU and reactivating one that was present before but was
temporarily disabled?
Doesn't cpu_up know which data structures get freed when an
active CPU is temporarily deactivated?
Doesn't cpu_down know what memory gets allocated in cpu_up?
Can't it deallocate just the right parts for the type of
transition it is doing?
It sounds like you're really asking how to make sure that cpu_up and
cpu_down both know what the other is doing, so that each can do the
opposite of the other. That doesn't sound hard.
Alan Stern
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