[RFC PATCH 04/16] x86/split_lock: Use non locked bit set instruction in set_cpu_cap

From: Fenghua Yu <fenghua.yu@intel.com>
To: "Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@elte.hu>,
	"H. Peter Anvin" <hpa@linux.intel.com>
Cc: "Ashok Raj" <ashok.raj@intel.com>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Rafael Wysocki" <rafael.j.wysocki@intel.com>,
	"Tony Luck" <tony.luck@intel.com>,
	"Alan Cox" <alan@linux.intel.com>,
	"Ravi V Shankar" <ravi.v.shankar@intel.com>,
	"Arjan van de Ven" <arjan@infradead.org>,
	"linux-kernel" <linux-kernel@vger.kernel.org>,
	"x86" <x86@kernel.org>, Fenghua Yu <fenghua.yu@intel.com>
Subject: [RFC PATCH 04/16] x86/split_lock: Use non locked bit set instruction in set_cpu_cap
Date: Sun, 27 May 2018 08:45:53 -0700	[thread overview]
Message-ID: <1527435965-202085-5-git-send-email-fenghua.yu@intel.com> (raw)
In-Reply-To: <1527435965-202085-1-git-send-email-fenghua.yu@intel.com>

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@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.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
