seqlock-serialize-against-writers.patch

From 6e204123c49ca166b4227fa64c36a32ac0091a96 Mon Sep 17 00:00:00 2001
From: Gregory Haskins <ghaskins@novell.com>
Date: Fri, 3 Jul 2009 08:44:28 -0500
Subject: [PATCH] seqlock: serialize against writers

commit 01c7b4356a44817a0a7027ca46371b2c7355d9c0 in tip.

There are currently several problems in -rt w.r.t. seqlock objects. RT
moves mainline seqlock_t to "raw_seqlock_t", and creates a new seqlock_t
object that is fully preemptible.  Being preemptible is a great step
towards deterministic behavior, but there are a few areas that need
additional measures to protect new vulnerabilities created by the
preemptible code. For the purposes of demonstration, consider three tasks
of different priority: A, B, and C.  A is the logically highest, and C is
the lowest.  A is trying to acquire a seqlock read critical section, while
C is involved in write locks.

Problem 1) If A spins in seqbegin due to writer contention retries, it may
prevent C from running even if C currently holds the write lock.  This
is a deadlock.

Problem 2) B may preempt C, preventing it from releasing the write
critical section.  In this case, A becomes inverted behind B.

Problem 3) Lower priority tasks such as C may continually acquire the write
section which subsequently causes A to continually retry and thus fail to
make forward progress.  Since C is lower priority it ideally should not
cause delays in A. E.g. C should block if A is in a read-lock and C is <= A.

This patch addresses Problems 1 & 2, and leaves 3 for a later time.

This patch changes the internal seqlock_t implementation to substitute
a rwlock for the basic spinlock previously used, and forces the readers
to serialize with the writers under contention.  Blocking on the read_lock
simultaneously sleeps A (preventing problem 1), while boosting C to A's
priority (preventing problem 2).  Non reader-to-writer contended
acquisitions, which are the predominant mode, remain free of atomic
operations.  Therefore the fast path should not be perturbed by this
change.

This fixes a real-world deadlock discovered under testing where all
high priority readers were hogging the cpus and preventing a writer
from releasing the lock (i.e. problem 1).

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 0c38f7c..a6de405 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -3,9 +3,11 @@
 /*
  * Reader/writer consistent mechanism without starving writers. This type of
  * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes.  Readers never
- * block but they may have to retry if a writer is in
- * progress. Writers do not wait for readers. 
+ * and is willing to retry if the information changes. Readers block
+ * on write contention (and where applicable, pi-boost the writer).
+ * Readers without contention on entry acquire the critical section
+ * without any atomic operations, but they may have to retry if a writer
+ * enters before the critical section ends. Writers do not wait for readers.
  *
  * This is not as cache friendly as brlock. Also, this will not work
  * for data that contains pointers, because any writer could
@@ -24,6 +26,8 @@
  *
  * Based on x86_64 vsyscall gettimeofday 
  * by Keith Owens and Andrea Arcangeli
+ *
+ * Priority inheritance and live-lock avoidance by Gregory Haskins
  */
 
 #include <linux/spinlock.h>
@@ -36,7 +40,7 @@ typedef struct {
 
 typedef struct {
 	unsigned sequence;
-	spinlock_t lock;
+	rwlock_t lock;
 } seqlock_t;
 
 /*
@@ -56,7 +60,7 @@ typedef struct {
 	raw_seqlock_t x = __RAW_SEQLOCK_UNLOCKED(x)
 
 #define __SEQLOCK_UNLOCKED(lockname) \
-	{ 0, __SPIN_LOCK_UNLOCKED(lockname) }
+	{ 0, __RW_LOCK_UNLOCKED(lockname) }
 
 #define SEQLOCK_UNLOCKED \
 	__SEQLOCK_UNLOCKED(old_style_seqlock_init)
@@ -64,7 +68,7 @@ typedef struct {
 #define seqlock_init(x)					\
 	do {						\
 		(x)->sequence = 0;			\
-		spin_lock_init(&(x)->lock);		\
+		rwlock_init(&(x)->lock);		\
 	} while (0)
 
 #define DEFINE_SEQLOCK(x) \
@@ -83,7 +87,7 @@ static inline void write_raw_seqlock(raw_seqlock_t *sl)
 
 static inline void write_seqlock(seqlock_t *sl)
 {
-	spin_lock(&sl->lock);
+	write_lock(&sl->lock);
 	++sl->sequence;
 	smp_wmb();
 }
@@ -99,12 +103,12 @@ static inline void write_sequnlock(seqlock_t *sl)
 {
 	smp_wmb();
 	sl->sequence++;
-	spin_unlock(&sl->lock);
+	write_unlock(&sl->lock);
 }
 
 static inline int write_tryseqlock(seqlock_t *sl)
 {
-	int ret = spin_trylock(&sl->lock);
+	int ret = write_trylock(&sl->lock);
 
 	if (ret) {
 		++sl->sequence;
@@ -129,18 +133,26 @@ repeat:
 	return ret;
 }
 
-static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
+static __always_inline unsigned read_seqbegin(seqlock_t *sl)
 {
 	unsigned ret;
 
-repeat:
 	ret = sl->sequence;
 	smp_rmb();
 	if (unlikely(ret & 1)) {
 		cpu_relax();
-		goto repeat;
+		/*
+		 * Serialze with the writer which will ensure they are
+		 * pi-boosted if necessary and prevent us from starving
+		 * them.
+		 */
+		read_lock(&sl->lock);
+		ret = sl->sequence;
+		read_unlock(&sl->lock);
 	}
 
+	BUG_ON(ret & 1);
+
 	return ret;
 }
 
-- 
1.7.1.1