linux-kernel - [PATCH v2] bit_spinlock: introduce smp_cond_load

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Date:   Tue, 30 Oct 2018 14:04:41 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Will Deacon" <will.deacon@....com>,
        <linux-kernel@...r.kernel.org>, Miao Xie <miaoxie@...wei.com>,
        Chao Yu <chao@...nel.org>, Gao Xiang <gaoxiang25@...wei.com>
Subject: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

It is better to use wrapped smp_cond_load_relaxed
instead of open-coded busy waiting for bit_spinlock.

Signed-off-by: Gao Xiang <gaoxiang25@...wei.com>
---

change log v2:
 - fix the incorrect expression !(VAL >> (bitnum & (BITS_PER_LONG-1)))
 - the test result is described in the following reply.

Thanks,
Gao Xiang

 include/linux/bit_spinlock.h | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index bbc4730a6505..d5f922b5ffd9 100644
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -15,22 +15,19 @@
  */
 static inline void bit_spin_lock(int bitnum, unsigned long *addr)
 {
-	/*
-	 * Assuming the lock is uncontended, this never enters
-	 * the body of the outer loop. If it is contended, then
-	 * within the inner loop a non-atomic test is used to
-	 * busywait with less bus contention for a good time to
-	 * attempt to acquire the lock bit.
-	 */
-	preempt_disable();
 #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
-	while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
-		preempt_enable();
-		do {
-			cpu_relax();
-		} while (test_bit(bitnum, addr));
+	const unsigned int bitshift = bitnum & (BITS_PER_LONG - 1);
+
+	while (1) {
+		smp_cond_load_relaxed(&addr[BIT_WORD(bitnum)],
+				      !((VAL >> bitshift) & 1));
 		preempt_disable();
+		if (!test_and_set_bit_lock(bitnum, addr))
+			break;
+		preempt_enable();
 	}
+#else
+	preempt_disable();
 #endif
 	__acquire(bitlock);
 }
-- 
2.17.1