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

From: tip-bot for Borislav Petkov
Date: Thu Mar 10 2016 - 06:08:42 EST

Commit-ID: 84477336ec03f8061ffd6908da341e063e5d6d1f
Gitweb: http://git.kernel.org/tip/84477336ec03f8061ffd6908da341e063e5d6d1f
Author: Borislav Petkov <bp@xxxxxxxxx>
AuthorDate: Wed, 9 Mar 2016 21:56:22 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 10 Mar 2016 11:27:12 +0100

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

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: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Huang Rui <ray.huang@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: spg_linux_kernel@xxxxxxx
Link: http://lkml.kernel.org/r/20160309205622.GG6564@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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 e912b2f..2f07c29 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