[PATCH] srcu-3: RCU variant permitting read-side blocking

Updated patch adding a variant of RCU that permits sleeping in read-side
critical sections.  SRCU is as follows:

o	Each use of SRCU creates its own srcu_struct, and each
	srcu_struct has its own set of grace periods.  This is
	critical, as it prevents one subsystem with a blocking
	reader from holding up SRCU grace periods for other
	subsystems.

o	The SRCU primitives (srcu_read_lock(), srcu_read_unlock(),
	and synchronize_srcu()) all take a pointer to a srcu_struct.

o	The SRCU primitives must be called from process context.

o	srcu_read_lock() returns an int that must be passed to
	the matching srcu_read_unlock().  Realtime RCU avoids the
	need for this by storing the state in the task struct,
	but SRCU needs to allow a given code path to pass through
	multiple SRCU domains -- storing state in the task struct
	would therefore require either arbitrary space in the
	task struct or arbitrary limits on SRCU nesting.  So I
	kicked the state-storage problem up to the caller.

	Of course, it is not permitted to call synchronize_srcu()
	while in an SRCU read-side critical section.

o	There is no call_srcu().  It would not be hard to implement
	one, but it seems like too easy a way to OOM the system.
	(Hey, we have enough trouble with call_rcu(), which does
	-not- permit readers to sleep!!!)  So, if you want it,
	please tell me why...

[josht@us.ibm.com: sparse notation]
Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 49 insertions, 0 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
new file mode 100644
index 000000000000..947fdab2ddb0
--- /dev/null
+++ b/include/linux/srcu.h
@@ -0,0 +1,49 @@
+/*
+ * Sleepable Read-Copy Update mechanism for mutual exclusion
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright (C) IBM Corporation, 2006
+ *
+ * Author: Paul McKenney <paulmck@us.ibm.com>
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ *              Documentation/RCU/ *.txt
+ *
+ */
+
+struct srcu_struct_array {
+        int c[2];
+};
+
+struct srcu_struct {
+        int completed;
+        struct srcu_struct_array *per_cpu_ref;
+        struct mutex mutex;
+};
+
+#ifndef CONFIG_PREEMPT
+#define srcu_barrier() barrier()
+#else /* #ifndef CONFIG_PREEMPT */
+#define srcu_barrier()
+#endif /* #else #ifndef CONFIG_PREEMPT */
+
+void init_srcu_struct(struct srcu_struct *sp);
+void cleanup_srcu_struct(struct srcu_struct *sp);
+int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
+void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
+void synchronize_srcu(struct srcu_struct *sp);
+long srcu_batches_completed(struct srcu_struct *sp);
+void cleanup_srcu_struct(struct srcu_struct *sp);