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
next prev 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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