Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Gao Xiang <gaoxiang25@huawei.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        <devel@driverdev.osuosl.org>
CC:     LKML <linux-kernel@vger.kernel.org>,
        <linux-erofs@lists.ozlabs.org>, "Chao Yu" <yuchao0@huawei.com>,
        Miao Xie <miaoxie@huawei.com>, <weidu.du@huawei.com>,
        Gao Xiang <gaoxiang25@huawei.com>
Subject: [PATCH 03/10] staging: erofs: atomic_cond_read_relaxed on ref-locked workgroup
Date:   Tue, 20 Nov 2018 22:34:18 +0800
Message-ID: <20181120143425.43637-4-gaoxiang25@huawei.com>
In-Reply-To: <20181120143425.43637-1-gaoxiang25@huawei.com>
References: <20181120143425.43637-1-gaoxiang25@huawei.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

It's better to use atomic_cond_read_relaxed, which is implemented
in hardware instructions to monitor a variable changes currently
for ARM64, instead of open-coded busy waiting.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
 drivers/staging/erofs/internal.h | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/drivers/staging/erofs/internal.h b/drivers/staging/erofs/internal.h
index 89dbd0888e53..eb80ba44d072 100644
--- a/drivers/staging/erofs/internal.h
+++ b/drivers/staging/erofs/internal.h
@@ -221,23 +221,29 @@ static inline void erofs_workgroup_unfreeze(
 	preempt_enable();
 }
 
+#if defined(CONFIG_SMP)
+static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
+{
+	return atomic_cond_read_relaxed(&grp->refcount,
+					VAL != EROFS_LOCKED_MAGIC);
+}
+#else
+static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
+{
+	int v = atomic_read(&grp->refcount);
+
+	/* workgroup is never freezed on uniprocessor systems */
+	DBG_BUGON(v == EROFS_LOCKED_MAGIC);
+	return v;
+}
+#endif
+
 static inline bool erofs_workgroup_get(struct erofs_workgroup *grp, int *ocnt)
 {
-	const int locked = (int)EROFS_LOCKED_MAGIC;
 	int o;
 
 repeat:
-	o = atomic_read(&grp->refcount);
-
-	/* spin if it is temporarily locked at the reclaim path */
-	if (unlikely(o == locked)) {
-#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
-		do
-			cpu_relax();
-		while (atomic_read(&grp->refcount) == locked);
-#endif
-		goto repeat;
-	}
+	o = erofs_wait_on_workgroup_freezed(grp);
 
 	if (unlikely(o <= 0))
 		return -1;
-- 
2.14.4