percpu_ref: make INIT_ATOMIC and switch_to_atomic() sticky
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>
Reviewed-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index ed280fb..6111bcb 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -67,6 +67,8 @@
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
@@ -202,7 +204,8 @@
* 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
@@ -216,6 +219,7 @@
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);
}
@@ -255,7 +259,10 @@
*
* 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
@@ -263,6 +270,8 @@
*/
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);
@@ -304,8 +313,8 @@
* @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.
@@ -316,6 +325,7 @@
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);