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SubjectRe: qemu:arm test failure due to commit 8053871d0f7f (smp: Fix smp_call_function_single_async() locking)
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On Sat, Apr 18, 2015 at 7:40 PM, Guenter Roeck <linux@roeck-us.net> wrote:
> On Sat, Apr 18, 2015 at 04:23:25PM -0700, Guenter Roeck wrote:
>>
>> my qemu test for arm:vexpress fails with the latest upstream kernel. It fails
>> hard - I don't get any output from the console. Bisect points to commit
>> 8053871d0f7f ("smp: Fix smp_call_function_single_async() locking").
>> Reverting this commit fixes the problem.

Hmm. It being qemu, can you look at where it seems to lock?

> Additional observation: The system boots if I add "-smp cpus=4" to the qemu
> options. It does still hang, however, with "-smp cpus=2" and "-smp cpus=3".

Funky.

That patch still looks obviously correct to me after looking at it
again, but I guess we need to revert it if somebody can't see what's
wrong.

It does make async (wait=0) smp_call_function_single() possibly be
*really* asynchronous, ie the 'csd' ends up being released and can be
re-used even before the call-single function has completed. That
should be a good thing, but I wonder if that triggers some ARM bug.

Instead of doing a full revert, what happens if you replace this part:

+ /* Do we wait until *after* callback? */
+ if (csd->flags & CSD_FLAG_SYNCHRONOUS) {
+ func(info);
+ csd_unlock(csd);
+ } else {
+ csd_unlock(csd);
+ func(info);
+ }

with just

+ func(info);
+ csd_unlock(csd);

ie keeping the csd locked until the function has actually completed? I
guess for completeness, we should do the same thing for the cpu ==
smp_processor_id() case (see the "We can unlock early" comment).

Now, if that makes a difference, I think it implies a bug in the
caller, so it's not the right fix, but it would be an interesting
thing to test.

Linus


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