Commit 8117ab50 authored by Ahmed S. Darwish's avatar Ahmed S. Darwish Committed by Peter Zijlstra

seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support

Preemption must be disabled before entering a sequence counter write
side critical section.  Otherwise the read side section can preempt the
write side section and spin for the entire scheduler tick.  If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.

Disabling preemption cannot be done for PREEMPT_RT though: it can lead
to higher latencies, and the write side sections will not be able to
acquire locks which become sleeping locks (e.g. spinlock_t).

To remain preemptible, while avoiding a possible livelock caused by the
reader preempting the writer, use a different technique: let the reader
detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
case, acquire then release the associated LOCKNAME writer serialization
lock. This will allow any possibly-preempted writer to make progress
until the end of its writer serialization lock critical section.

Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
an associated (PREEMPT_RT) sleeping lock.

References: 55f3560d ("seqlock: Extend seqcount API with associated locks")
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/20200519214547.352050-1-a.darwish@linutronix.de
parent 52ac39e5
......@@ -17,6 +17,7 @@
#include <linux/kcsan-checks.h>
#include <linux/lockdep.h>
#include <linux/mutex.h>
#include <linux/ww_mutex.h>
#include <linux/preempt.h>
#include <linux/spinlock.h>
......@@ -131,7 +132,23 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* See Documentation/locking/seqlock.rst
*/
#ifdef CONFIG_LOCKDEP
/*
* For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
* disable preemption. It can lead to higher latencies, and the write side
* sections will not be able to acquire locks which become sleeping locks
* (e.g. spinlock_t).
*
* To remain preemptible while avoiding a possible livelock caused by the
* reader preempting the writer, use a different technique: let the reader
* detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
* case, acquire then release the associated LOCKNAME writer serialization
* lock. This will allow any possibly-preempted writer to make progress
* until the end of its writer serialization lock critical section.
*
* This lock-unlock technique must be implemented for all of PREEMPT_RT
* sleeping locks. See Documentation/locking/locktypes.rst
*/
#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
#define __SEQ_LOCK(expr) expr
#else
#define __SEQ_LOCK(expr)
......@@ -162,10 +179,12 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
* @locktype: LOCKNAME canonical C data type
* @preemptible: preemptibility of above lockname
* @preemptible: preemptibility of above locktype
* @lockmember: argument for lockdep_assert_held()
* @lockbase: associated lock release function (prefix only)
* @lock_acquire: associated lock acquisition function (full call)
*/
#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember) \
#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
typedef struct seqcount_##lockname { \
seqcount_t seqcount; \
__SEQ_LOCK(locktype *lock); \
......@@ -187,13 +206,33 @@ __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
static __always_inline unsigned \
__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
{ \
return READ_ONCE(s->seqcount.sequence); \
unsigned seq = READ_ONCE(s->seqcount.sequence); \
\
if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
return seq; \
\
if (preemptible && unlikely(seq & 1)) { \
__SEQ_LOCK(lock_acquire); \
__SEQ_LOCK(lockbase##_unlock(s->lock)); \
\
/* \
* Re-read the sequence counter since the (possibly \
* preempted) writer made progress. \
*/ \
seq = READ_ONCE(s->seqcount.sequence); \
} \
\
return seq; \
} \
\
static __always_inline bool \
__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
{ \
if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
return preemptible; \
\
/* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
return false; \
} \
\
static __always_inline void \
......@@ -226,11 +265,13 @@ static inline void __seqprop_assert(const seqcount_t *s)
lockdep_assert_preemption_disabled();
}
SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock)
SEQCOUNT_LOCKNAME(spinlock, spinlock_t, false, s->lock)
SEQCOUNT_LOCKNAME(rwlock, rwlock_t, false, s->lock)
SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock)
SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base)
#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
/*
* SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
......
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