Commit 1909760f authored by Ahmed S. Darwish's avatar Ahmed S. Darwish Committed by Peter Zijlstra
Browse files

seqlock: PREEMPT_RT: Do not starve seqlock_t writers 



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: default avatarAhmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200904153231.11994-6-a.darwish@linutronix.de
parent 8117ab50

include/linux/seqlock.h

+21 −11
Original line number Diff line number Diff line
@@ -790,13 +790,17 @@ static inline void raw_write_seqcount_latch(seqcount_latch_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)			\

	}
@@ -806,8 +810,8 @@ 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)



/**
@@ -854,6 +858,12 @@ 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
@@ -870,7 +880,7 @@ 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);

}



/**
@@ -882,7 +892,7 @@ 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);

}
@@ -896,7 +906,7 @@ 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);

}



/**
@@ -909,7 +919,7 @@ 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);

}
@@ -923,7 +933,7 @@ 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);

}



/**
@@ -935,7 +945,7 @@ 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);

}
@@ -944,7 +954,7 @@ 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;

}
@@ -973,7 +983,7 @@ 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);

}

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