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authorTejun Heo <tj@kernel.org>2014-09-24 13:31:49 -0400
committerTejun Heo <tj@kernel.org>2014-09-24 13:31:49 -0400
commitf47ad45784611297b699f3dffb6c7222b76afe64 (patch)
tree9497c0a4bad4c6b97cfd08bf22a8b2313606ce6a /lib
parent490c79a65708873228cf114cf00e32c204e4e907 (diff)
percpu_ref: decouple switching to percpu mode and reinit
percpu_ref has treated the dropping of the base reference and switching to atomic mode as an integral operation; however, there's nothing inherent tying the two together. The use cases for percpu_ref have been expanding continuously. While the current init/kill/reinit/exit model can cover a lot, the coupling of kill/reinit with atomic/percpu mode switching is turning out to be too restrictive for use cases where many percpu_refs are created and destroyed back-to-back with only some of them reaching extended operation. The coupling also makes implementing always-atomic debug mode difficult. This patch separates out percpu mode switching into percpu_ref_switch_to_percpu() and reimplements percpu_ref_reinit() on top of it. * DEAD still requires ATOMIC. A dead ref can't be switched to percpu mode w/o going through reinit. v2: __percpu_ref_switch_to_percpu() was missing static. Fixed. Reported by Fengguang aka kbuild test robot. 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> Cc: kbuild test robot <fengguang.wu@intel.com>
Diffstat (limited to 'lib')
-rw-r--r--lib/percpu-refcount.c73
1 files changed, 54 insertions, 19 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 6e0d14366c5d..5a6d43baccc5 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -206,40 +206,54 @@ void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
__percpu_ref_switch_to_atomic(ref, confirm_switch);
}
-/**
- * percpu_ref_reinit - re-initialize a percpu refcount
- * @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, killed
- * and reached 0 but not exited.
- *
- * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
- * this function is in progress.
- */
-void percpu_ref_reinit(struct percpu_ref *ref)
+static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
int cpu;
BUG_ON(!percpu_count);
- WARN_ON_ONCE(!percpu_ref_is_zero(ref));
- atomic_long_set(&ref->count, 1 + PERCPU_COUNT_BIAS);
+ if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
+ return;
+
+ wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
+
+ atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
/*
* Restore per-cpu operation. smp_store_release() is paired with
* smp_read_barrier_depends() in __ref_is_percpu() and guarantees
* that the zeroing is visible to all percpu accesses which can see
- * the following __PERCPU_REF_ATOMIC_DEAD clearing.
+ * the following __PERCPU_REF_ATOMIC clearing.
*/
for_each_possible_cpu(cpu)
*per_cpu_ptr(percpu_count, cpu) = 0;
smp_store_release(&ref->percpu_count_ptr,
- ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
+ ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
+}
+
+/**
+ * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
+ * @ref: percpu_ref to switch to percpu mode
+ *
+ * There's no reason to use this function for the usual reference counting.
+ * To re-use an expired ref, use percpu_ref_reinit().
+ *
+ * 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.
+ *
+ * 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
+ * by percpu_ref_switch_atomic().
+ */
+void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
+{
+ /* 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);
}
-EXPORT_SYMBOL_GPL(percpu_ref_reinit);
/**
* percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
@@ -253,8 +267,8 @@ EXPORT_SYMBOL_GPL(percpu_ref_reinit);
* percpu_ref_tryget_live() for details.
*
* 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
- * the process of switching to atomic mode by percpu_ref_switch_atomic().
+ * but it may block if @confirm_kill is specified and @ref is in the
+ * process of switching to atomic mode by percpu_ref_switch_atomic().
*
* Due to the way percpu_ref is implemented, @confirm_switch will be called
* after at least one full sched RCU grace period has passed but this is an
@@ -271,3 +285,24 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
percpu_ref_put(ref);
}
EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
+
+/**
+ * percpu_ref_reinit - re-initialize a percpu refcount
+ * @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.
+ *
+ * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
+ * this function is in progress.
+ */
+void percpu_ref_reinit(struct percpu_ref *ref)
+{
+ WARN_ON_ONCE(!percpu_ref_is_zero(ref));
+
+ ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
+ percpu_ref_get(ref);
+ __percpu_ref_switch_to_percpu(ref);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_reinit);

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