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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>,
        "Ashok Raj" <ashok.raj@intel.com>,
        "Ravi V Shankar" <ravi.v.shankar@intel.com>,
        "Tony Luck" <tony.luck@intel.com>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Rafael Wysocki" <rafael.j.wysocki@intel.com>,
        "Arjan van de Ven" <arjan@infradead.org>,
        "Alan Cox" <alan@linux.intel.com>
Cc:     "x86" <x86@kernel.org>,
        "linux-kernel" <linux-kernel@vger.kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>
Subject: [PATCH 04/15] x86/split_lock: Use non locked bit set instruction in set_cpu_cap
Date:   Mon, 14 May 2018 11:52:14 -0700
Message-Id: <1526323945-211107-5-git-send-email-fenghua.yu@intel.com>
In-Reply-To: <1526323945-211107-1-git-send-email-fenghua.yu@intel.com>
References: <1526323945-211107-1-git-send-email-fenghua.yu@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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

Since the c->x86_capability is not aligned to cache line depending on
compiler, the locked bit set instruction falls into split lock
situation which causes #AC exception when #AC exception is enabled by
split locked accesses.

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

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

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 b27da9602a6d..604f4c514efa 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