percpu: implement generic percpu refcounting

This implements a refcount with similar semantics to
atomic_get()/atomic_dec_and_test() - but percpu.

It also implements two stage shutdown, as we need it to tear down the
percpu counts.  Before dropping the initial refcount, you must call
percpu_ref_kill(); this puts the refcount in "shutting down mode" and
switches back to a single atomic refcount with the appropriate
barriers (synchronize_rcu()).

It's also legal to call percpu_ref_kill() multiple times - it only
returns true once, so callers don't have to reimplement shutdown
synchronization.

[akpm@linux-foundation.org: fix build]
[akpm@linux-foundation.org: coding-style tweak]
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Zach Brown <zab@redhat.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Asai Thambi S P <asamymuthupa@micron.com>
Cc: Selvan Mani <smani@micron.com>
Cc: Sam Bradshaw <sbradshaw@micron.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Reviewed-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/percpu-refcount.h

@@ -0,0 +1,122 @@
+/*
+ * 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;
+
+	preempt_disable();
+
+	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);
+
+	preempt_enable();
+}
+
+/**
+ * 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;
+
+	preempt_disable();
+
+	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);
+
+	preempt_enable();
+}
+
+#endif