mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-09-12 05:48:40 +00:00
![Tejun Heo](/assets/img/avatar_default.png)
percpu_ref_get/put() are using preempt_disable/enable() while percpu_ref_kill() is using plain call_rcu() instead of call_rcu_sched(). This is buggy as grace periods of the two may not match. Fix it by using plain RCU in percpu_ref_get/put(). (I suggested using sched RCU in the first place but there's no actual benefit in doing so unless we're gonna introduce different variants of get/put to be called while preemption is alredy disabled, which we definitely shouldn't.) Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Rusty Russell <rusty@rustcorp.com.au> Acked-by: Kent Overstreet <koverstreet@google.com>
122 lines
3.8 KiB
C
122 lines
3.8 KiB
C
/*
|
|
* Percpu refcounts:
|
|
* (C) 2012 Google, Inc.
|
|
* Author: Kent Overstreet <koverstreet@google.com>
|
|
*
|
|
* This implements a refcount with similar semantics to atomic_t - atomic_inc(),
|
|
* atomic_dec_and_test() - but percpu.
|
|
*
|
|
* There's one important difference between percpu refs and normal atomic_t
|
|
* refcounts; you have to keep track of your initial refcount, and then when you
|
|
* start shutting down you call percpu_ref_kill() _before_ dropping the initial
|
|
* refcount.
|
|
*
|
|
* The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
|
|
* than an atomic_t - this is because of the way shutdown works, see
|
|
* percpu_ref_kill()/PCPU_COUNT_BIAS.
|
|
*
|
|
* Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
|
|
* refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
|
|
* puts the ref back in single atomic_t mode, collecting the per cpu refs and
|
|
* issuing the appropriate barriers, and then marks the ref as shutting down so
|
|
* that percpu_ref_put() will check for the ref hitting 0. After it returns,
|
|
* it's safe to drop the initial ref.
|
|
*
|
|
* USAGE:
|
|
*
|
|
* See fs/aio.c for some example usage; it's used there for struct kioctx, which
|
|
* is created when userspaces calls io_setup(), and destroyed when userspace
|
|
* calls io_destroy() or the process exits.
|
|
*
|
|
* In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
|
|
* calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove
|
|
* the kioctx from the proccess's list of kioctxs - after that, there can't be
|
|
* any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
|
|
* the initial ref with percpu_ref_put().
|
|
*
|
|
* Code that does a two stage shutdown like this often needs some kind of
|
|
* explicit synchronization to ensure the initial refcount can only be dropped
|
|
* once - percpu_ref_kill() does this for you, it returns true once and false if
|
|
* someone else already called it. The aio code uses it this way, but it's not
|
|
* necessary if the code has some other mechanism to synchronize teardown.
|
|
* around.
|
|
*/
|
|
|
|
#ifndef _LINUX_PERCPU_REFCOUNT_H
|
|
#define _LINUX_PERCPU_REFCOUNT_H
|
|
|
|
#include <linux/atomic.h>
|
|
#include <linux/kernel.h>
|
|
#include <linux/percpu.h>
|
|
#include <linux/rcupdate.h>
|
|
|
|
struct percpu_ref;
|
|
typedef void (percpu_ref_release)(struct percpu_ref *);
|
|
|
|
struct percpu_ref {
|
|
atomic_t count;
|
|
/*
|
|
* The low bit of the pointer indicates whether the ref is in percpu
|
|
* mode; if set, then get/put will manipulate the atomic_t (this is a
|
|
* hack because we need to keep the pointer around for
|
|
* percpu_ref_kill_rcu())
|
|
*/
|
|
unsigned __percpu *pcpu_count;
|
|
percpu_ref_release *release;
|
|
struct rcu_head rcu;
|
|
};
|
|
|
|
int percpu_ref_init(struct percpu_ref *, percpu_ref_release *);
|
|
void percpu_ref_kill(struct percpu_ref *ref);
|
|
|
|
#define PCPU_STATUS_BITS 2
|
|
#define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1)
|
|
#define PCPU_REF_PTR 0
|
|
#define PCPU_REF_DEAD 1
|
|
|
|
#define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK)
|
|
|
|
/**
|
|
* percpu_ref_get - increment a percpu refcount
|
|
*
|
|
* Analagous to atomic_inc().
|
|
*/
|
|
static inline void percpu_ref_get(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count;
|
|
|
|
rcu_read_lock();
|
|
|
|
pcpu_count = ACCESS_ONCE(ref->pcpu_count);
|
|
|
|
if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
|
|
__this_cpu_inc(*pcpu_count);
|
|
else
|
|
atomic_inc(&ref->count);
|
|
|
|
rcu_read_unlock();
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_put - decrement a percpu refcount
|
|
*
|
|
* Decrement the refcount, and if 0, call the release function (which was passed
|
|
* to percpu_ref_init())
|
|
*/
|
|
static inline void percpu_ref_put(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count;
|
|
|
|
rcu_read_lock();
|
|
|
|
pcpu_count = ACCESS_ONCE(ref->pcpu_count);
|
|
|
|
if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
|
|
__this_cpu_dec(*pcpu_count);
|
|
else if (unlikely(atomic_dec_and_test(&ref->count)))
|
|
ref->release(ref);
|
|
|
|
rcu_read_unlock();
|
|
}
|
|
|
|
#endif
|