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

[Guenter Roeck]

On 04/19/2015 02:31 AM, Ingo Molnar wrote:
> * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > On Sun, Apr 19, 2015 at 4:48 AM, Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > Does that smaller patch work equally well?
> >
> > .. and here's a properly formatted email and patch.
> >
> >             Linus
>
> >   kernel/smp.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 2aaac2c47683..07854477c164 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -159,8 +159,10 @@ static int generic_exec_single(int cpu, struct call_single_data *csd,
> >   	}
> >
> >
> > -	if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu))
> > +	if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu)) {
> > +		csd_unlock(csd);
> >   		return -ENXIO;
> > +	}
>
> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>

> Btw., in this case we should probably also generate a WARN_ONCE()
> warning?
>
> I _think_ most such callers calling an SMP function call for offline
> or out of range CPUs are at minimum racy.
Not really; at least the online cpu part is an absolutely normal use
case for qemu-arm.

Sure, you can argue that "this isn't the real system", and that
qemu-arm should be "fixed", but there are reasons - the emulation
is (much) slower if the number of CPUs is set to 4, and not everyone
who wants to use qemu has a system with as many CPUs as the emulated
system would normally have.

Thanks,
Guenter

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