locking: Move the percpu-rwsem code to kernel/locking/

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-52bjmtty46we26hbfd9sc9iy@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 167 insertions, 1 deletions

diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index bdd313a3411d..baab8e5e7f66 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -21,4 +21,5 @@ obj-$(CONFIG_RT_MUTEX_TESTER) += rtmutex-tester.o
 obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
 obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock_debug.o
 obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
-obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
\ No newline at end of file
+obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
+obj-$(CONFIG_PERCPU_RWSEM) += percpu-rwsem.o
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
new file mode 100644
index 000000000000..652a8ee8efe9
--- /dev/null
+++ b/kernel/locking/percpu-rwsem.c
@@ -0,0 +1,165 @@
+#include <linux/atomic.h>
+#include <linux/rwsem.h>
+#include <linux/percpu.h>
+#include <linux/wait.h>
+#include <linux/lockdep.h>
+#include <linux/percpu-rwsem.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/errno.h>
+
+int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
+                        const char *name, struct lock_class_key *rwsem_key)
+{
+        brw->fast_read_ctr = alloc_percpu(int);
+        if (unlikely(!brw->fast_read_ctr))
+                return -ENOMEM;
+
+        /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
+        __init_rwsem(&brw->rw_sem, name, rwsem_key);
+        atomic_set(&brw->write_ctr, 0);
+        atomic_set(&brw->slow_read_ctr, 0);
+        init_waitqueue_head(&brw->write_waitq);
+        return 0;
+}
+
+void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
+{
+        free_percpu(brw->fast_read_ctr);
+        brw->fast_read_ctr = NULL; /* catch use after free bugs */
+}
+
+/*
+ * This is the fast-path for down_read/up_read, it only needs to ensure
+ * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
+ * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
+ * serialize with the preempt-disabled section below.
+ *
+ * The nontrivial part is that we should guarantee acquire/release semantics
+ * in case when
+ *
+ *      R_W: down_write() comes after up_read(), the writer should see all
+ *           changes done by the reader
+ * or
+ *      W_R: down_read() comes after up_write(), the reader should see all
+ *           changes done by the writer
+ *
+ * If this helper fails the callers rely on the normal rw_semaphore and
+ * atomic_dec_and_test(), so in this case we have the necessary barriers.
+ *
+ * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
+ * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
+ * reader inside the critical section. See the comments in down_write and
+ * up_write below.
+ */
+static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
+{
+        bool success = false;
+
+        preempt_disable();
+        if (likely(!atomic_read(&brw->write_ctr))) {
+                __this_cpu_add(*brw->fast_read_ctr, val);
+                success = true;
+        }
+        preempt_enable();
+
+        return success;
+}
+
+/*
+ * Like the normal down_read() this is not recursive, the writer can
+ * come after the first percpu_down_read() and create the deadlock.
+ *
+ * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
+ * percpu_up_read() does rwsem_release(). This pairs with the usage
+ * of ->rw_sem in percpu_down/up_write().
+ */
+void percpu_down_read(struct percpu_rw_semaphore *brw)
+{
+        might_sleep();
+        if (likely(update_fast_ctr(brw, +1))) {
+                rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+                return;
+        }
+
+        down_read(&brw->rw_sem);
+        atomic_inc(&brw->slow_read_ctr);
+        /* avoid up_read()->rwsem_release() */
+        __up_read(&brw->rw_sem);
+}
+
+void percpu_up_read(struct percpu_rw_semaphore *brw)
+{
+        rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
+
+        if (likely(update_fast_ctr(brw, -1)))
+                return;
+
+        /* false-positive is possible but harmless */
+        if (atomic_dec_and_test(&brw->slow_read_ctr))
+                wake_up_all(&brw->write_waitq);
+}
+
+static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
+{
+        unsigned int sum = 0;
+        int cpu;
+
+        for_each_possible_cpu(cpu) {
+                sum += per_cpu(*brw->fast_read_ctr, cpu);
+                per_cpu(*brw->fast_read_ctr, cpu) = 0;
+        }
+
+        return sum;
+}
+
+/*
+ * A writer increments ->write_ctr to force the readers to switch to the
+ * slow mode, note the atomic_read() check in update_fast_ctr().
+ *
+ * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
+ * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
+ * counter it represents the number of active readers.
+ *
+ * Finally the writer takes ->rw_sem for writing and blocks the new readers,
+ * then waits until the slow counter becomes zero.
+ */
+void percpu_down_write(struct percpu_rw_semaphore *brw)
+{
+        /* tell update_fast_ctr() there is a pending writer */
+        atomic_inc(&brw->write_ctr);
+        /*
+         * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
+         *    so that update_fast_ctr() can't succeed.
+         *
+         * 2. Ensures we see the result of every previous this_cpu_add() in
+         *    update_fast_ctr().
+         *
+         * 3. Ensures that if any reader has exited its critical section via
+         *    fast-path, it executes a full memory barrier before we return.
+         *    See R_W case in the comment above update_fast_ctr().
+         */
+        synchronize_sched_expedited();
+
+        /* exclude other writers, and block the new readers completely */
+        down_write(&brw->rw_sem);
+
+        /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
+        atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
+
+        /* wait for all readers to complete their percpu_up_read() */
+        wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
+}
+
+void percpu_up_write(struct percpu_rw_semaphore *brw)
+{
+        /* release the lock, but the readers can't use the fast-path */
+        up_write(&brw->rw_sem);
+        /*
+         * Insert the barrier before the next fast-path in down_read,
+         * see W_R case in the comment above update_fast_ctr().
+         */
+        synchronize_sched_expedited();
+        /* the last writer unblocks update_fast_ctr() */
+        atomic_dec(&brw->write_ctr);
+}