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authorTejun Heo <tj@kernel.org>2015-09-29 17:47:20 -0400
committerTejun Heo <tj@kernel.org>2016-08-10 15:02:58 -0400
commit33e465ce7cb30b71c113a26f36d293b545a28e12 (patch)
treef090e719daf982ca77d1b129fa1667bf3021c5e9 /lib
parent3f49bdd95855a33eea749304d2e10530a869218b (diff)
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percpu_ref: allow operation mode switching operations to be called concurrently
percpu_ref initially didn't have explicit mode switching operations. It started out in percpu mode and switched to atomic mode on kill and then released. Ensuring that kill operation is initiated only after init completes was naturally the caller's responsibility. percpu_ref_reinit() was introduced later but it didn't shift the synchronization responsibility. Reinit can't be performed until kill is confirmed, so there was nothing to worry about synchronization-wise. Also, as both reinit and kill manipulate the base reference, invocations of the same function couldn't be allowed to race each other. The latest additions of percpu_ref_switch_to_atomic/percpu() changed the situation. These two functions can be called any time as long as the percpu_ref is between init and exit and thus there are valid valid usage scenarios where these new functions race with each other or against reinit/kill. Mostly from inertia, f47ad4578461 ("percpu_ref: decouple switching to percpu mode and reinit") still left synchronization among percpu mode switching operations to its users. That the new switch functions can be freely mixed with kill/reinit but the operations themselves should be synchronized is too subtle a requirement and led to a very subtle race condition in blk-mq freezing path. This patch fixes the situation by introducing percpu_ref_switch_lock to protect mode switching operations. This ensures that percpu-ref users don't have to worry about mode changing operations racing against each other, e.g. switch_to_percpu against kill, as long as the sequence of operations is valid. Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Akinobu Mita <akinobu.mita@gmail.com> Link: http://lkml.kernel.org/g/1443287365-4244-7-git-send-email-akinobu.mita@gmail.com Fixes: f47ad4578461 ("percpu_ref: decouple switching to percpu mode and reinit")
Diffstat (limited to 'lib')
-rw-r--r--lib/percpu-refcount.c33
1 files changed, 29 insertions, 4 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index f3ff793691ac..c69938e4b0d5 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -33,6 +33,7 @@
#define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
+static DEFINE_SPINLOCK(percpu_ref_switch_lock);
static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
@@ -208,15 +209,15 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
static void __percpu_ref_switch_mode(struct percpu_ref *ref,
percpu_ref_func_t *confirm_switch)
{
+ lockdep_assert_held(&percpu_ref_switch_lock);
+
/*
* 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);
+ wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
+ percpu_ref_switch_lock);
if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
__percpu_ref_switch_to_atomic(ref, confirm_switch);
@@ -247,8 +248,14 @@ static void __percpu_ref_switch_mode(struct percpu_ref *ref,
void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
percpu_ref_func_t *confirm_switch)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
ref->force_atomic = true;
__percpu_ref_switch_mode(ref, confirm_switch);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
}
/**
@@ -271,8 +278,14 @@ void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
*/
void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
ref->force_atomic = false;
__percpu_ref_switch_mode(ref, NULL);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
}
/**
@@ -293,12 +306,18 @@ void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
percpu_ref_func_t *confirm_kill)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
"%s called more than once on %pf!", __func__, ref->release);
ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
__percpu_ref_switch_mode(ref, confirm_kill);
percpu_ref_put(ref);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
}
EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
@@ -315,10 +334,16 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
*/
void percpu_ref_reinit(struct percpu_ref *ref)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
WARN_ON_ONCE(!percpu_ref_is_zero(ref));
ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
percpu_ref_get(ref);
__percpu_ref_switch_mode(ref, NULL);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
}
EXPORT_SYMBOL_GPL(percpu_ref_reinit);