[PATCH 4/5] percpu_ref: restructure operation mode switching

[Tejun Heo <tj@kernel.org>]

Restructure atomic/percpu mode switching.

* The users of __percpu_ref_switch_to_atomic/percpu() now call a new
  function __percpu_ref_switch_mode() which calls either of the
  original switching functions depending on the current state of
  ref->force_atomic and the __PERCPU_REF_DEAD flag.  The callers no
  longer check whether switching is necessary but always invoke
  __percpu_ref_switch_mode().

* !ref->confirm_switch waiting is collected into
  __percpu_ref_switch_mode().

This patch doesn't cause any behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 lib/percpu-refcount.c | 64 +++++++++++++++++++++++----------------------------
 1 file changed, 29 insertions(+), 35 deletions(-)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 6a36597..1e69c3b 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -161,16 +161,6 @@ static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 					  percpu_ref_func_t *confirm_switch)
 {
-	/*
-	 * If the previous ATOMIC switching hasn't finished yet, wait for
-	 * its completion.  If the caller ensures that ATOMIC switching
-	 * isn't in progress, this function can be called from any context.
-	 * Do an extra confirm_switch test to circumvent the unconditional
-	 * might_sleep() in wait_event().
-	 */
-	if (ref->confirm_switch)
-		wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
-
 	if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
 		if (confirm_switch)
 			confirm_switch(ref);
@@ -195,16 +185,6 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 	int cpu;
 
-	/*
-	 * If the previous ATOMIC switching hasn't finished yet, wait for
-	 * its completion.  If the caller ensures that ATOMIC switching
-	 * isn't in progress, this function can be called from any context.
-	 * Do an extra confirm_switch test to circumvent the unconditional
-	 * might_sleep() in wait_event().
-	 */
-	if (ref->confirm_switch)
-		wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
-
 	BUG_ON(!percpu_count);
 
 	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
@@ -225,6 +205,25 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 			  ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
 }
 
+static void __percpu_ref_switch_mode(struct percpu_ref *ref,
+				     percpu_ref_func_t *confirm_switch)
+{
+	/*
+	 * If the previous ATOMIC switching hasn't finished yet, wait for
+	 * its completion.  If the caller ensures that ATOMIC switching
+	 * isn't in progress, this function can be called from any context.
+	 * Do an extra confirm_switch test to circumvent the unconditional
+	 * might_sleep() in wait_event().
+	 */
+	if (ref->confirm_switch)
+		wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
+
+	if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
+		__percpu_ref_switch_to_atomic(ref, confirm_switch);
+	else
+		__percpu_ref_switch_to_percpu(ref);
+}
+
 /**
  * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
  * @ref: percpu_ref to switch to atomic mode
@@ -241,16 +240,15 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
  * operations.  Note that @ref will stay in atomic mode across kill/reinit
  * cycles until percpu_ref_switch_to_percpu() is called.
  *
- * This function normally doesn't block and can be called from any context
- * but it may block if @confirm_kill is specified and @ref is already in
- * the process of switching to atomic mode.  In such cases, @confirm_switch
- * will be invoked after the switching is complete.
+ * This function may block if @ref is in the process of switching to atomic
+ * mode.  If the caller ensures that @ref is not in the process of
+ * switching to atomic mode, this function can be called from any context.
  */
 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_switch)
 {
 	ref->force_atomic = true;
-	__percpu_ref_switch_to_atomic(ref, confirm_switch);
+	__percpu_ref_switch_mode(ref, confirm_switch);
 }
 
 /**
@@ -267,17 +265,14 @@ void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
  * dying or dead, the actual switching takes place on the following
  * percpu_ref_reinit().
  *
- * This function normally doesn't block and can be called from any context
- * but it may block if @ref is in the process of switching to atomic mode
- * by percpu_ref_switch_atomic().
+ * This function may block if @ref is in the process of switching to atomic
+ * mode.  If the caller ensures that @ref is not in the process of
+ * switching to atomic mode, this function can be called from any context.
  */
 void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 {
 	ref->force_atomic = false;
-
-	/* a dying or dead ref can't be switched to percpu mode w/o reinit */
-	if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD))
-		__percpu_ref_switch_to_percpu(ref);
+	__percpu_ref_switch_mode(ref, NULL);
 }
 
 /**
@@ -302,7 +297,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 		  "%s called more than once on %pf!", __func__, ref->release);
 
 	ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
-	__percpu_ref_switch_to_atomic(ref, confirm_kill);
+	__percpu_ref_switch_mode(ref, confirm_kill);
 	percpu_ref_put(ref);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
@@ -324,7 +319,6 @@ void percpu_ref_reinit(struct percpu_ref *ref)
 
 	ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
 	percpu_ref_get(ref);
-	if (!ref->force_atomic)
-		__percpu_ref_switch_to_percpu(ref);
+	__percpu_ref_switch_mode(ref, NULL);
 }
 EXPORT_SYMBOL_GPL(percpu_ref_reinit);
-- 
2.4.3