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Subject[PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage
Date
Proper documentation for the design and usage of sequence counters and
sequential locks does not exist. Complete the seqlock.h documentation as
follows:

- Divide all documentation on a seqcount_t vs. seqlock_t basis. The
description for both mechanisms was intermingled, which is incorrect
since the usage constrains for each type are vastly different.

- Add an introductory paragraph describing the internal design of, and
rationale for, sequence counters.

- Document seqcount_t writer non-preemptibility requirement, which was
not previously documented anywhere, and provide a clear rationale.

- Provide template code for seqcount_t and seqlock_t initialization
and reader/writer critical sections.

- Recommend using seqlock_t by default. It implicitly handles the
serialization and non-preemptibility requirements of writers.

At seqlock.h:

- Remove references to brlocks as they've long been removed from the
kernel.

- Remove references to gcc-3.x since the kernel's minimum supported
gcc version is 4.6.

- Remove the severely lacking top comment and reference the newly
introduced Documentation/locking/seqlock.rst file instead.

References: 0f6ed63b1707 ("no need to keep brlock macros anymore...")
References: cafa0010cd51 ("Raise the minimum required gcc version to 4.6")
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
Documentation/locking/index.rst | 1 +
Documentation/locking/seqlock.rst | 181 ++++++++++++++++++++++++++++++
include/linux/seqlock.h | 73 +++++-------
3 files changed, 213 insertions(+), 42 deletions(-)
create mode 100644 Documentation/locking/seqlock.rst

diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
index 5d6800a723dc..aad15fc81ccd 100644
--- a/Documentation/locking/index.rst
+++ b/Documentation/locking/index.rst
@@ -14,6 +14,7 @@ locking
mutex-design
rt-mutex-design
rt-mutex
+ seqlock
spinlocks
ww-mutex-design

diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
new file mode 100644
index 000000000000..2242ae00e7bf
--- /dev/null
+++ b/Documentation/locking/seqlock.rst
@@ -0,0 +1,181 @@
+======================================
+Sequence counters and sequential locks
+======================================
+
+Introduction
+============
+
+Sequence counters are a reader-writer consistency mechanism with
+lockless readers (read-only retry loops), and no writer starvation. They
+are used for data that's rarely written to (e.g. system time), where the
+reader wants a consistent set of information and is willing to retry if
+that information changes.
+
+A data set is consistent when the sequence count at the beginning of the
+read side critical section is even and the same sequence count value is
+read again at the end of the critical section. The data in the set must
+be copied out inside the read side critical section. If the sequence
+count has changed between the start and the end of the critical section,
+the reader must retry.
+
+Writers increment the sequence count at the start and the end of their
+critical section. After starting the critical section the sequence count
+is odd and indicates to the readers that an update is in progress. At
+the end of the write side critical section the sequence count becomes
+even again which lets readers make progress.
+
+A sequence counter write side critical section must never be preempted
+or interrupted by read side sections. Otherwise the reader will spin for
+the entire scheduler tick due to the odd sequence count value and the
+interrupted writer. If that reader belongs to a real-time scheduling
+class, it can spin forever and the kernel will livelock.
+
+.. _seqcount_t:
+
+Sequence counters (:c:type:`seqcount_t`)
+========================================
+
+This is the the raw counting mechanism, which does not protect against
+multiple writers. Write side critical sections must thus be serialized
+by an external lock.
+
+If the write serialization primitive is not implicitly disabling
+preemption, preemption must be explicitly disabled before entering the
+write side section. If the sequence counter read section can be invoked
+from hardirq or softirq contexts, interrupts or bottom halves must be
+respectively disabled before entering the write side section.
+
+If it's desired to automatically handle the sequence counter
+requirements of writer serialization and non-preemptibility, use a
+:ref:`sequential lock <seqlock_t>` instead.
+
+Initialization:
+
+.. code-block:: c
+
+ /* dynamic */
+ seqcount_t foo_seqcount;
+ seqcount_init(&foo_seqcount);
+
+ /* static */
+ static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
+
+ /* C99 struct init */
+ struct {
+ .seq = SEQCNT_ZERO(foo.seq),
+ } foo;
+
+Write path:
+
+.. code-block:: c
+
+ /* Serialized context with disabled preemption */
+
+ write_seqcount_begin(&foo_seqcount);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_seqcount_end(&foo_seqcount);
+
+Read path:
+
+.. code-block:: c
+
+ do {
+ seq = read_seqcount_begin(&foo_seqcount);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqcount_retry(&foo_seqcount, seq));
+
+.. _seqlock_t:
+
+Sequential locks (:c:type:`seqlock_t`)
+======================================
+
+This contains the :ref:`sequence counting mechanism <seqcount_t>`
+earlier discussed, plus an embedded spinlock for writer serialization
+and non-preemptibility.
+
+If the read side section can be invoked from hardirq or softirq context,
+use the write side function variants which respectively disable
+interrupts or bottom halves.
+
+Initialization:
+
+.. code-block:: c
+
+ /* dynamic */
+ seqlock_t foo_seqlock;
+ seqlock_init(&foo_seqlock);
+
+ /* static */
+ static DEFINE_SEQLOCK(foo_seqlock);
+
+ /* C99 struct init */
+ struct {
+ .seql = __SEQLOCK_UNLOCKED(foo.seql)
+ } foo;
+
+Write path:
+
+.. code-block:: c
+
+ write_seqlock(&foo_seqlock);
+
+ /* ... [[write-side critical section]] ... */
+
+ write_sequnlock(&foo_seqlock);
+
+Read path, three categories:
+
+1. Normal Sequence readers which never block a writer but they must
+ retry if a writer is in progress by detecting change in the sequence
+ number. Writers do not wait for a sequence reader.
+
+ .. code-block:: c
+
+ do {
+ seq = read_seqbegin(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (read_seqretry(&foo_seqlock, seq));
+
+2. Locking readers which will wait if a writer or another locking reader
+ is in progress. A locking reader in progress will also block a writer
+ from entering its critical section. This read lock is
+ exclusive. Unlike rwlock_t, only one locking reader can acquire it.
+
+ .. code-block:: c
+
+ read_seqlock_excl(&foo_seqlock);
+
+ /* ... [[read-side critical section]] ... */
+
+ read_sequnlock_excl(&foo_seqlock);
+
+3. Conditional lockless reader (as in 1), or locking reader (as in 2),
+ according to a passed marker. This is used to avoid lockless readers
+ starvation (too much retry loops) in case of a sharp spike in write
+ activity. First, a lockless read is tried (even marker passed). If
+ that trial fails (odd sequence counter is returned, which is used as
+ the next iteration marker), the lockless read is transformed to a
+ full locking read and no retry loop is necessary.
+
+ .. code-block:: c
+
+ /* marker; even initialization */
+ int seq = 0;
+ do {
+ read_seqbegin_or_lock(&foo_seqlock, &seq);
+
+ /* ... [[read-side critical section]] ... */
+
+ } while (need_seqretry(&foo_seqlock, seq));
+ done_seqretry(&foo_seqlock, seq);
+
+API documentation
+=================
+
+.. kernel-doc:: include/linux/seqlock.h
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d35be7709403..2a4af746b1da 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1,36 +1,15 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
+
/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
+ * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
+ * lockless readers (read-only retry loops), and no writer starvation.
*
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
+ * See Documentation/locking/seqlock.rst for full description.
*
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
- *
- *
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
+ * Copyrights:
+ * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
*/

#include <linux/spinlock.h>
@@ -40,11 +19,23 @@
#include <asm/processor.h>

/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
+ * Sequence counters (seqcount_t)
+ *
+ * The raw counting mechanism without any writer protection. Write side
+ * critical sections must be serialized and readers on the same CPU
+ * (e.g. through preemption or interrupts) must be excluded.
+ *
+ * If the write serialization mechanism is one of the common kernel
+ * locking primitives, use a sequence counter with associated lock
+ * (seqcount_LOCKTYPE_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
@@ -221,8 +212,6 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
return __read_seqcount_retry(s, start);
}

-
-
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
s->sequence++;
@@ -367,11 +356,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
smp_wmb(); /* increment "sequence" before following stores */
}

-/*
- * Sequence counter only version assumes that callers are using their
- * own locking and preemption is disabled.
- */
-
static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
@@ -419,15 +403,20 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
s->sequence+=2;
}

+/*
+ * Sequential locks (seqlock_t)
+ *
+ * Sequence counters with an embedded spinlock for writer serialization
+ * and non-preemptibility.
+ *
+ * See Documentation/locking/seqlock.rst
+ */
+
typedef struct {
struct seqcount seqcount;
spinlock_t lock;
} seqlock_t;

-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
--
2.20.1
\
 
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