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>

lib/percpu-refcount.c

@@ -0,0 +1,128 @@
+#define pr_fmt(fmt) "%s: " fmt "\n", __func__
+
+#include <linux/kernel.h>
+#include <linux/percpu-refcount.h>
+
+/*
+ * Initially, a percpu refcount is just a set of percpu counters. Initially, we
+ * don't try to detect the ref hitting 0 - which means that get/put can just
+ * increment or decrement the local counter. Note that the counter on a
+ * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
+ * percpu counters will all sum to the correct value
+ *
+ * (More precisely: because moduler arithmatic is commutative the sum of all the
+ * pcpu_count vars will be equal to what it would have been if all the gets and
+ * puts were done to a single integer, even if some of the percpu integers
+ * overflow or underflow).
+ *
+ * The real trick to implementing percpu refcounts is shutdown. We can't detect
+ * the ref hitting 0 on every put - this would require global synchronization
+ * and defeat the whole purpose of using percpu refs.
+ *
+ * What we do is require the user to keep track of the initial refcount; we know
+ * the ref can't hit 0 before the user drops the initial ref, so as long as we
+ * convert to non percpu mode before the initial ref is dropped everything
+ * works.
+ *
+ * Converting to non percpu mode is done with some RCUish stuff in
+ * percpu_ref_kill. Additionally, we need a bias value so that the atomic_t
+ * can't hit 0 before we've added up all the percpu refs.
+ */
+
+#define PCPU_COUNT_BIAS		(1U << 31)
+
+/**
+ * percpu_ref_init - initialize a percpu refcount
+ * @ref:	ref to initialize
+ * @release:	function which will be called when refcount hits 0
+ *
+ * Initializes the refcount in single atomic counter mode with a refcount of 1;
+ * analagous to atomic_set(ref, 1).
+ *
+ * Note that @release must not sleep - it may potentially be called from RCU
+ * callback context by percpu_ref_kill().
+ */
+int percpu_ref_init(struct percpu_ref *ref, percpu_ref_release *release)
+{
+	atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
+
+	ref->pcpu_count = alloc_percpu(unsigned);
+	if (!ref->pcpu_count)
+		return -ENOMEM;
+
+	ref->release = release;
+	return 0;
+}
+
+static void percpu_ref_kill_rcu(struct rcu_head *rcu)
+{
+	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
+	unsigned __percpu *pcpu_count;
+	unsigned count = 0;
+	int cpu;
+
+	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
+
+	/* Mask out PCPU_REF_DEAD */
+	pcpu_count = (unsigned __percpu *)
+		(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
+
+	for_each_possible_cpu(cpu)
+		count += *per_cpu_ptr(pcpu_count, cpu);
+
+	free_percpu(pcpu_count);
+
+	pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
+
+	/*
+	 * It's crucial that we sum the percpu counters _before_ adding the sum
+	 * to &ref->count; since gets could be happening on one cpu while puts
+	 * happen on another, adding a single cpu's count could cause
+	 * @ref->count to hit 0 before we've got a consistent value - but the
+	 * sum of all the counts will be consistent and correct.
+	 *
+	 * Subtracting the bias value then has to happen _after_ adding count to
+	 * &ref->count; we need the bias value to prevent &ref->count from
+	 * reaching 0 before we add the percpu counts. But doing it at the same
+	 * time is equivalent and saves us atomic operations:
+	 */
+
+	atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
+
+	/*
+	 * Now we're in single atomic_t mode with a consistent refcount, so it's
+	 * safe to drop our initial ref:
+	 */
+	percpu_ref_put(ref);
+}
+
+/**
+ * percpu_ref_kill - safely drop initial ref
+ *
+ * Must be used to drop the initial ref on a percpu refcount; must be called
+ * precisely once before shutdown.
+ *
+ * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
+ * percpu counters and dropping the initial ref.
+ */
+void percpu_ref_kill(struct percpu_ref *ref)
+{
+	unsigned __percpu *pcpu_count, *old, *new;
+
+	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
+
+	do {
+		if (REF_STATUS(pcpu_count) == PCPU_REF_DEAD) {
+			WARN(1, "percpu_ref_kill() called more than once!\n");
+			return;
+		}
+
+		old = pcpu_count;
+		new = (unsigned __percpu *)
+			(((unsigned long) pcpu_count)|PCPU_REF_DEAD);
+
+		pcpu_count = cmpxchg(&ref->pcpu_count, old, new);
+	} while (pcpu_count != old);
+
+	call_rcu(&ref->rcu, percpu_ref_kill_rcu);
+}