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From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	"Sebastian A. Siewior" <bigeasy@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>
Subject: [PATCH v2 5/5] seqlock: PREEMPT_RT: Do not starve seqlock_t writers
Date: Fri,  4 Sep 2020 17:32:31 +0200
Message-ID: <20200904153231.11994-6-a.darwish@linutronix.de> (raw)
In-Reply-To: <20200904153231.11994-1-a.darwish@linutronix.de>

On PREEMPT_RT, seqlock_t is transformed to a sleeping lock that do not
disable preemption. A seqlock_t reader can thus preempt its write side
section and spin for the enter scheduler tick. If that reader belongs to
a real-time scheduling class, it can spin forever and the kernel will
livelock.

To break this livelock possibility on PREEMPT_RT, implement seqlock_t in
terms of "seqcount_spinlock_t" instead of plain "seqcount_t".

Beside its pure annotational value, this will leverage the existing
seqcount_LOCKNAME_T PREEMPT_RT anti-livelock mechanisms, without adding
any extra code.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 include/linux/seqlock.h | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 6ac5a63fc536..06a339355c3a 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -757,13 +757,17 @@ static inline void raw_write_seqcount_t_latch(seqcount_t *s)
  *    - Documentation/locking/seqlock.rst
  */
 typedef struct {
-	struct seqcount seqcount;
+	/*
+	 * Make sure that readers don't starve writers on PREEMPT_RT: use
+	 * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
+	 */
+	seqcount_spinlock_t seqcount;
 	spinlock_t lock;
 } seqlock_t;
 
 #define __SEQLOCK_UNLOCKED(lockname)					\
 	{								\
-		.seqcount = SEQCNT_ZERO(lockname),			\
+		.seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
 		.lock =	__SPIN_LOCK_UNLOCKED(lockname)			\
 	}
 
@@ -773,8 +777,8 @@ typedef struct {
  */
 #define seqlock_init(sl)						\
 	do {								\
-		seqcount_init(&(sl)->seqcount);				\
 		spin_lock_init(&(sl)->lock);				\
+		seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);	\
 	} while (0)
 
 /**
@@ -821,6 +825,12 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 	return read_seqcount_retry(&sl->seqcount, start);
 }
 
+/*
+ * For all seqlock_t write side functions, use write_seqcount_*t*_begin()
+ * instead of the generic write_seqcount_begin(). This way, no redundant
+ * lockdep_assert_held() checks are added.
+ */
+
 /**
  * write_seqlock() - start a seqlock_t write side critical section
  * @sl: Pointer to seqlock_t
@@ -837,7 +847,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 static inline void write_seqlock(seqlock_t *sl)
 {
 	spin_lock(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -849,7 +859,7 @@ static inline void write_seqlock(seqlock_t *sl)
  */
 static inline void write_sequnlock(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock(&sl->lock);
 }
 
@@ -863,7 +873,7 @@ static inline void write_sequnlock(seqlock_t *sl)
 static inline void write_seqlock_bh(seqlock_t *sl)
 {
 	spin_lock_bh(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -876,7 +886,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
  */
 static inline void write_sequnlock_bh(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_bh(&sl->lock);
 }
 
@@ -890,7 +900,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
 static inline void write_seqlock_irq(seqlock_t *sl)
 {
 	spin_lock_irq(&sl->lock);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 }
 
 /**
@@ -902,7 +912,7 @@ static inline void write_seqlock_irq(seqlock_t *sl)
  */
 static inline void write_sequnlock_irq(seqlock_t *sl)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_irq(&sl->lock);
 }
 
@@ -911,7 +921,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 	unsigned long flags;
 
 	spin_lock_irqsave(&sl->lock, flags);
-	write_seqcount_t_begin(&sl->seqcount);
+	write_seqcount_t_begin(&sl->seqcount.seqcount);
 	return flags;
 }
 
@@ -940,7 +950,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 static inline void
 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 {
-	write_seqcount_t_end(&sl->seqcount);
+	write_seqcount_t_end(&sl->seqcount.seqcount);
 	spin_unlock_irqrestore(&sl->lock, flags);
 }
 
-- 
2.28.0


  parent reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-04 15:32 [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 1/5] seqlock: seqcount_LOCKNAME_t: Standardize naming convention Ahmed S. Darwish
2020-09-10 15:08   ` [tip: locking/core] " tip-bot2 for Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 2/5] seqlock: Use unique prefix for seqcount_t property accessors Ahmed S. Darwish
2020-09-08 11:41   ` peterz
2020-09-10 15:08   ` [tip: locking/core] " tip-bot2 for Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 3/5] seqlock: seqcount_t: Implement all read APIs as statement expressions Ahmed S. Darwish
2020-09-10 15:08   ` [tip: locking/core] " tip-bot2 for Ahmed S. Darwish
2020-09-04 15:32 ` [PATCH v2 4/5] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support Ahmed S. Darwish
2020-09-08 11:45   ` peterz
2020-09-08 12:48     ` Ahmed S. Darwish
2020-09-04 15:32 ` Ahmed S. Darwish [this message]
2020-09-10 15:08   ` [tip: locking/core] seqlock: PREEMPT_RT: Do not starve seqlock_t writers tip-bot2 for Ahmed S. Darwish
2020-09-15  0:20 ` [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support Qian Cai
2020-09-15 12:48   ` Boqun Feng
2020-09-15 13:10     ` Boqun Feng
2020-09-15 14:30     ` peterz
2020-09-16 12:52       ` Qian Cai
2020-09-16 12:54         ` peterz
2020-09-16 13:00           ` Qian Cai
2020-09-16 13:02             ` peterz
2020-09-17  2:31               ` Stephen Rothwell
2020-09-18  8:42       ` [tip: locking/core] seqlock: Unbreak lockdep tip-bot2 for peterz@infradead.org

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