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. Link: https://lkml.kernel.org/r/159708609435.2571.13948681727529247231.tglx@nanos Link: https://lkml.kernel.org/r/[email protected] References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks") Signed-off-by: Ahmed S. Darwish <[email protected]> --- include/linux/seqlock.h | 72 +++++++++++++++++++++++++++++++++-------- 1 file changed, 58 insertions(+), 14 deletions(-) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index ed1c6c0ff8bb..6ac5a63fc536 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -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); \ @@ -187,7 +206,23 @@ __seqprop_seqcount_##lockname##_ptr(seqcount_##lockname##_t *s) \ static __always_inline unsigned \ __seqprop_seqcount_##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 \ @@ -226,11 +261,13 @@ static inline void __seqprop_seqcount_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 @@ -406,13 +443,20 @@ static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start) return __read_seqcount_t_retry(s, start); } +/* + * Enforce non-preemptibility for all seqcount_LOCKNAME_t writers. Don't + * do it for PREEMPT_RT, for the reasons outlined at __SEQ_LOCK(). + */ +#define __seq_enforce_writer_non_preemptibility(s) \ + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s)) + /** * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants */ #define raw_write_seqcount_begin(s) \ do { \ - if (__seqcount_lock_preemptible(s)) \ + if (__seq_enforce_writer_non_preemptibility(s)) \ preempt_disable(); \ \ raw_write_seqcount_t_begin(__seqcount_ptr(s)); \ @@ -433,7 +477,7 @@ static inline void raw_write_seqcount_t_begin(seqcount_t *s) do { \ raw_write_seqcount_t_end(__seqcount_ptr(s)); \ \ - if (__seqcount_lock_preemptible(s)) \ + if (__seq_enforce_writer_non_preemptibility(s)) \ preempt_enable(); \ } while (0) @@ -456,7 +500,7 @@ static inline void raw_write_seqcount_t_end(seqcount_t *s) do { \ __seqcount_assert_lock_held(s); \ \ - if (__seqcount_lock_preemptible(s)) \ + if (__seq_enforce_writer_non_preemptibility(s)) \ preempt_disable(); \ \ write_seqcount_t_begin_nested(__seqcount_ptr(s), subclass); \ @@ -483,7 +527,7 @@ static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass) do { \ __seqcount_assert_lock_held(s); \ \ - if (__seqcount_lock_preemptible(s)) \ + if (__seq_enforce_writer_non_preemptibility(s)) \ preempt_disable(); \ \ write_seqcount_t_begin(__seqcount_ptr(s)); \ @@ -504,7 +548,7 @@ static inline void write_seqcount_t_begin(seqcount_t *s) do { \ write_seqcount_t_end(__seqcount_ptr(s)); \ \ - if (__seqcount_lock_preemptible(s)) \ + if (__seq_enforce_writer_non_preemptibility(s)) \ preempt_enable(); \ } while (0) -- 2.28.0
