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percpu_ref: make INIT_ATOMIC and switch_to_atomic() sticky

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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>
This commit is contained in:
Tejun Heo 2014-09-24 13:31:50 -04:00
parent 2aad2a86f6
commit 1cae13e75b
2 changed files with 19 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
include/linux/percpu-refcount.h
View file

@ -67,7 +67,9 @@ 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,
@ -87,6 +89,7 @@ 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;
};

20
lib/percpu-refcount.c
View file

@ -67,6 +67,8 @@ 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
@ -202,7 +204,8 @@ 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
@ -216,6 +219,7 @@ 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);
}
@ -255,7 +259,10 @@ static 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
@ -263,6 +270,8 @@ static 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);
@ -304,8 +313,8 @@ 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.
@ -316,6 +325,7 @@ 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);