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

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Date:	Thu, 10 Mar 2016 03:07:24 -0800
From:	tip-bot for Borislav Petkov <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, luto@...capital.net, luto@...nel.org,
	hpa@...or.com, torvalds@...ux-foundation.org, mingo@...nel.org,
	bp@...en8.de, bp@...e.de, ray.huang@....com, peterz@...radead.org,
	tglx@...utronix.de, a.p.zijlstra@...llo.nl
Subject: [tip:x86/urgent] x86/delay: Avoid preemptible context checks in
 delay_mwaitx()

Commit-ID:  84477336ec03f8061ffd6908da341e063e5d6d1f
Gitweb:     http://git.kernel.org/tip/84477336ec03f8061ffd6908da341e063e5d6d1f
Author:     Borislav Petkov <bp@...en8.de>
AuthorDate: Wed, 9 Mar 2016 21:56:22 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
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@...nel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Huang Rui <ray.huang@....com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: spg_linux_kernel@....com
Link: http://lkml.kernel.org/r/20160309205622.GG6564@pd.tnic
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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