linux-kernel - [RFC PATCH 04/16] x86/split_lock: Use non locked bit set instruction in set_cpu

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Date:   Sun, 27 May 2018 08:45:53 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...e.hu>,
        "H. Peter Anvin" <hpa@...ux.intel.com>
Cc:     "Ashok Raj" <ashok.raj@...el.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Rafael Wysocki" <rafael.j.wysocki@...el.com>,
        "Tony Luck" <tony.luck@...el.com>,
        "Alan Cox" <alan@...ux.intel.com>,
        "Ravi V Shankar" <ravi.v.shankar@...el.com>,
        "Arjan van de Ven" <arjan@...radead.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "x86" <x86@...nel.org>, Fenghua Yu <fenghua.yu@...el.com>
Subject: [RFC PATCH 04/16] x86/split_lock: Use non locked bit set instruction in set_cpu_cap

set_bit() called by set_cpu_cap() is a locked bit set instruction for
atomic operation.

Since the c->x86_capability can span two cache lines depending on kernel
configuration and building evnironment, the locked bit set instruction may
cause #AC exception when #AC exception for split lock is enabled.

But set_cpu_cap() is only called in early init phase on one CPU and
therefore there is no contention for set_cpu_cap(). It's unnecessary
to be atomic operation.

Using __set_bit(), which is not a locked instruction, is faster than
the locked instruction and can fix kernel split lock issue.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Acked-by: Dave Hansen <dave.hansen@...el.com>
---
 arch/x86/include/asm/cpufeature.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index aced6c9290d6..0793ec95d18b 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -128,7 +128,8 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 
 #define boot_cpu_has(bit)	cpu_has(&boot_cpu_data, bit)
 
-#define set_cpu_cap(c, bit)	set_bit(bit, (unsigned long *)((c)->x86_capability))
+#define set_cpu_cap(c, bit)	\
+	__set_bit(bit, (unsigned long *)((c)->x86_capability))
 
 extern void setup_clear_cpu_cap(unsigned int bit);
 extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
-- 
2.5.0