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From:   Pavel Begunkov <asml.silence@gmail.com>
To:     linux-block@vger.kernel.org
Cc:     Jens Axboe <axboe@kernel.dk>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Christoph Lameter <cl@linux.com>,
        Tejun Heo <tj@kernel.org>, Dennis Zhou <dennis@kernel.org>,
        Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH v2 1/2] percpu_ref: percpu_ref_tryget_live() version holding RCU
Date:   Thu, 21 Oct 2021 14:30:51 +0100
Message-Id: <3066500d7a6eb3e03f10adf98b87fdb3b1c49db8.1634822969.git.asml.silence@gmail.com>
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References: <cover.1634822969.git.asml.silence@gmail.com>
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Add percpu_ref_tryget_live_rcu(), which is a version of
percpu_ref_tryget_live() but the user is responsible for enclosing it in
a RCU read lock section.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 include/linux/percpu-refcount.h | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index ae16a9856305..b31d3f3312ce 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -266,6 +266,28 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 	return percpu_ref_tryget_many(ref, 1);
 }
 
+/**
+ * percpu_ref_tryget_live_rcu - same as percpu_ref_tryget_live() but the
+ * caller is responsible for taking RCU.
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref)
+{
+	unsigned long __percpu *percpu_count;
+	bool ret = false;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	if (likely(__ref_is_percpu(ref, &percpu_count))) {
+		this_cpu_inc(*percpu_count);
+		ret = true;
+	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
+		ret = atomic_long_inc_not_zero(&ref->data->count);
+	}
+	return ret;
+}
+
 /**
  * percpu_ref_tryget_live - try to increment a live percpu refcount
  * @ref: percpu_ref to try-get
@@ -283,20 +305,11 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
  */
 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 {
-	unsigned long __percpu *percpu_count;
 	bool ret = false;
 
 	rcu_read_lock();
-
-	if (__ref_is_percpu(ref, &percpu_count)) {
-		this_cpu_inc(*percpu_count);
-		ret = true;
-	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
-		ret = atomic_long_inc_not_zero(&ref->data->count);
-	}
-
+	ret = percpu_ref_tryget_live_rcu(ref);
 	rcu_read_unlock();
-
 	return ret;
 }
 
-- 
2.33.1