[PATCH] x86/delay: Avoid preemptible context checks in delay_mwaitx()

From: Borislav Petkov
Date: Wed Mar 09 2016 - 15:56:47 EST

We do use this_cpu_ptr(&cpu_tss) as a cacheline-aligned, seldomly
accessed per-cpu var as the MONITORX target in delay_mwaitx(). However,
when called in preemptible context, this_cpu_ptr -> smp_processor_id() ->
debug_smp_processor_id() fires:

BUG: using smp_processor_id() in preemptible [00000000] code: udevd/312
caller is delay_mwaitx+0x40/0xa0

But we don't care about that check - we only need cpu_tss as a MONITORX
target and it doesn't really matter which CPU's var we're touching as
we're going idle anyway. Fix that.

Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Huang Rui <ray.huang@xxxxxxx>
Cc: spg_linux_kernel@xxxxxxx
arch/x86/lib/delay.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index e912b2f6d36e..2f07c291dcc8 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -102,7 +102,7 @@ static void delay_mwaitx(unsigned long __loops)
* Use cpu_tss as a cacheline-aligned, seldomly
* accessed per-cpu variable as the monitor target.
- __monitorx(this_cpu_ptr(&cpu_tss), 0, 0);
+ __monitorx(raw_cpu_ptr(&cpu_tss), 0, 0);

* AMD, like Intel, supports the EAX hint and EAX=0xf


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