[PATCH 9/9] percpu_ref: make INIT_ATOMIC and switch_to_atomic() sticky

Tejun Heo
2014-09-23T05:55:53Z

Currently, a percpu_ref which is initialized with
PERPCU_REF_INIT_ATOMIC or switched to atomic mode via
switch_to_atomic() automatically reverts to percpu mode on the first
percpu_ref_reinit(). This makes the atomic mode difficult to use for
cases where a percpu_ref is used as a persistent on/off switch which
may be cycled multiple times.

This patch makes such atomic state sticky so that it survives through
kill/reinit cycles. After this patch, atomic state is cleared only by
an explicit percpu_ref_switch_to_percpu() call.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
include/linux/percpu-refcount.h | 5 ++++-
lib/percpu-refcount.c | 20 +++++++++++++++-----
2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 5f84bf0..8459d3a 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -65,7 +65,9 @@ enum {
enum {
/*
* Start w/ ref == 1 in atomic mode. Can be switched to percpu
- * operation using percpu_ref_switch_to_percpu().
+ * operation using percpu_ref_switch_to_percpu(). If initialized
+ * with this flag, the ref will stay in atomic mode until
+ * percpu_ref_switch_to_percpu() is invoked on it.
*/
PERCPU_REF_INIT_ATOMIC = 1 << 0,

@@ -85,6 +87,7 @@ struct percpu_ref {
unsigned long percpu_count_ptr;
percpu_ref_func_t *release;
percpu_ref_func_t *confirm_switch;
+ bool force_atomic:1;
struct rcu_head rcu;
};

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 74ec33e..c47e496 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -68,6 +68,8 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
if (!ref->percpu_count_ptr)
return -ENOMEM;

+ ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
+
if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
else
@@ -203,7 +205,8 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
* are guaraneed to be in atomic mode, @confirm_switch, which may not
* block, is invoked. This function may be invoked concurrently with all
* the get/put operations and can safely be mixed with kill and reinit
- * operations.
+ * 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
@@ -217,6 +220,7 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
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);
}

@@ -256,7 +260,10 @@ void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
*
* Switch @ref to percpu mode. This function may be invoked concurrently
* with all the get/put operations and can safely be mixed with kill and
- * reinit operations.
+ * reinit operations. This function reverses the sticky atomic state set
+ * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
+ * 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
@@ -264,6 +271,8 @@ void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
*/
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);
@@ -305,8 +314,8 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
* @ref: perpcu_ref to re-initialize
*
* Re-initialize @ref so that it's in the same state as when it finished
- * percpu_ref_init(). @ref must have been initialized successfully and
- * reached 0 but not exited.
+ * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
+ * initialized successfully and reached 0 but not exited.
*
* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
* this function is in progress.
@@ -317,6 +326,7 @@ void percpu_ref_reinit(struct percpu_ref *ref)

ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
percpu_ref_get(ref);
- __percpu_ref_switch_to_percpu(ref);
+ if (!ref->force_atomic)
+ __percpu_ref_switch_to_percpu(ref);
}
EXPORT_SYMBOL_GPL(percpu_ref_reinit);
--
1.9.3