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Date: Mon, 18 Dec 2023 12:02:40 -0500
From: Waiman Long <longman@...hat.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-fsdevel@...r.kernel.org
Cc: tglx@...utronix.de, x86@...nel.org, tj@...nel.org, peterz@...radead.org,
mathieu.desnoyers@...icios.com, paulmck@...nel.org, keescook@...omium.org,
dave.hansen@...ux.intel.com, mingo@...hat.com, will@...nel.org,
boqun.feng@...il.com, brauner@...nel.org
Subject: Re: [PATCH 21/50] locking/seqlock: Split out seqlock_types.h
On 12/15/23 22:26, Kent Overstreet wrote:
> Trimming down sched.h dependencies: we don't want to include more than
> the base types.
>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> ---
> include/linux/sched.h | 2 +-
> include/linux/seqlock.h | 79 +----------------------------
> include/linux/seqlock_types.h | 93 +++++++++++++++++++++++++++++++++++
> 3 files changed, 96 insertions(+), 78 deletions(-)
> create mode 100644 include/linux/seqlock_types.h
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 6d803d0904d9..436f7ce1450a 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -33,7 +33,7 @@
> #include <linux/task_io_accounting.h>
> #include <linux/posix-timers_types.h>
> #include <linux/rseq.h>
> -#include <linux/seqlock.h>
> +#include <linux/seqlock_types.h>
> #include <linux/kcsan.h>
> #include <linux/rv.h>
> #include <linux/livepatch_sched.h>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index e92f9d5577ba..d90d8ee29d81 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -18,6 +18,7 @@
> #include <linux/lockdep.h>
> #include <linux/mutex.h>
> #include <linux/preempt.h>
> +#include <linux/seqlock_types.h>
> #include <linux/spinlock.h>
>
> #include <asm/processor.h>
> @@ -37,37 +38,6 @@
> */
> #define KCSAN_SEQLOCK_REGION_MAX 1000
>
> -/*
> - * Sequence counters (seqcount_t)
> - *
> - * This is the raw counting mechanism, without any writer protection.
> - *
> - * Write side critical sections must be serialized and non-preemptible.
> - *
> - * If readers can be invoked from hardirq or softirq contexts,
> - * interrupts or bottom halves must also be respectively disabled before
> - * entering the write section.
> - *
> - * This mechanism can't be used if the protected data contains pointers,
> - * as the writer can invalidate a pointer that a reader is following.
> - *
> - * If the write serialization mechanism is one of the common kernel
> - * locking primitives, use a sequence counter with associated lock
> - * (seqcount_LOCKNAME_t) instead.
> - *
> - * If it's desired to automatically handle the sequence counter writer
> - * serialization and non-preemptibility requirements, use a sequential
> - * lock (seqlock_t) instead.
> - *
> - * See Documentation/locking/seqlock.rst
> - */
> -typedef struct seqcount {
> - unsigned sequence;
> -#ifdef CONFIG_DEBUG_LOCK_ALLOC
> - struct lockdep_map dep_map;
> -#endif
> -} seqcount_t;
> -
> static inline void __seqcount_init(seqcount_t *s, const char *name,
> struct lock_class_key *key)
> {
> @@ -131,28 +101,6 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
> * See Documentation/locking/seqlock.rst
> */
>
> -/*
> - * 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)
> -#endif
> -
> /*
> * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
> * @seqcount: The real sequence counter
> @@ -194,11 +142,6 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
> * @lockbase: prefix for associated lock/unlock
> */
> #define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockbase) \
> -typedef struct seqcount_##lockname { \
> - seqcount_t seqcount; \
> - __SEQ_LOCK(locktype *lock); \
> -} seqcount_##lockname##_t; \
> - \
> static __always_inline seqcount_t * \
> __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
> { \
> @@ -284,6 +227,7 @@ SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, raw_spin)
> SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, spin)
> SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, read)
> SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
> +#undef SEQCOUNT_LOCKNAME
>
> /*
> * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
> @@ -794,25 +738,6 @@ static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
> smp_wmb(); /* increment "sequence" before following stores */
> }
>
> -/*
> - * Sequential locks (seqlock_t)
> - *
> - * Sequence counters with an embedded spinlock for writer serialization
> - * and non-preemptibility.
> - *
> - * For more info, see:
> - * - Comments on top of seqcount_t
> - * - Documentation/locking/seqlock.rst
> - */
> -typedef struct {
> - /*
> - * 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_SPINLOCK_ZERO(lockname, &(lockname).lock), \
> diff --git a/include/linux/seqlock_types.h b/include/linux/seqlock_types.h
> new file mode 100644
> index 000000000000..dfdf43e3fa3d
> --- /dev/null
> +++ b/include/linux/seqlock_types.h
> @@ -0,0 +1,93 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __LINUX_SEQLOCK_TYPES_H
> +#define __LINUX_SEQLOCK_TYPES_H
> +
> +#include <linux/lockdep_types.h>
> +#include <linux/mutex_types.h>
> +#include <linux/spinlock_types.h>
> +
> +/*
> + * Sequence counters (seqcount_t)
> + *
> + * This is the raw counting mechanism, without any writer protection.
> + *
> + * Write side critical sections must be serialized and non-preemptible.
> + *
> + * If readers can be invoked from hardirq or softirq contexts,
> + * interrupts or bottom halves must also be respectively disabled before
> + * entering the write section.
> + *
> + * This mechanism can't be used if the protected data contains pointers,
> + * as the writer can invalidate a pointer that a reader is following.
> + *
> + * If the write serialization mechanism is one of the common kernel
> + * locking primitives, use a sequence counter with associated lock
> + * (seqcount_LOCKNAME_t) instead.
> + *
> + * If it's desired to automatically handle the sequence counter writer
> + * serialization and non-preemptibility requirements, use a sequential
> + * lock (seqlock_t) instead.
> + *
> + * See Documentation/locking/seqlock.rst
> + */
> +typedef struct seqcount {
> + unsigned sequence;
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> + struct lockdep_map dep_map;
> +#endif
> +} seqcount_t;
> +
> +/*
> + * 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)
> +#endif
> +
> +#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockbase) \
> +typedef struct seqcount_##lockname { \
> + seqcount_t seqcount; \
> + __SEQ_LOCK(locktype *lock); \
> +} seqcount_##lockname##_t;
> +
> +SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, raw_spin)
> +SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, spin)
> +SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, read)
> +SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
> +#undef SEQCOUNT_LOCKNAME
> +
> +/*
> + * Sequential locks (seqlock_t)
> + *
> + * Sequence counters with an embedded spinlock for writer serialization
> + * and non-preemptibility.
> + *
> + * For more info, see:
> + * - Comments on top of seqcount_t
> + * - Documentation/locking/seqlock.rst
> + */
> +typedef struct {
> + /*
> + * 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;
> +
> +#endif /* __LINUX_SEQLOCK_TYPES_H */
seqlock.h is directly included in kernel/sched/sched.h, so breaking out
seqlock_types.h and including only that in include/linux/sched.h should
be OK.
Acked-by: Waiman Long <longman@...hat.com>
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