Re: qemu:arm test failure due to commit 8053871d0f7f (smp: Fix smp_call_function_single_async() locking)

[Linus Torvalds]

On Sat, Apr 18, 2015 at 7:40 PM, Guenter Roeck <linux@xxxxxxxxxxxx> 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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