linux-kernel - [PATCH] x86/delay: Avoid preemptible context checks in delay

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Message-ID: <20160309205622.GG6564@pd.tnic>
Date:	Wed, 9 Mar 2016 21:56:22 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Huang Rui <ray.huang@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	spg_linux_kernel@....com, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] 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@...nel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Huang Rui <ray.huang@....com>
Cc: spg_linux_kernel@....com
---
 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
-- 
2.3.5


-- 
Regards/Gruss,
    Boris.

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