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

From: Guenter Roeck
Date: Sun Apr 19 2015 - 10:09:18 EST


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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