Documentation/locking/seqlock.rst

======================================
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.

This mechanism cannot be used if the protected data contains pointers,
as the writer can invalidate a pointer that the reader is following.


.. _seqcount_t:

Sequence counters (``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 read section can be invoked from hardirq or
softirq contexts, interrupts or bottom halves must also be respectively
disabled before entering the write section.

If it's desired to automatically handle the sequence counter
requirements of writer serialization and non-preemptibility, use
:ref:`seqlock_t` instead.

Initialization::

	/* 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::

	/* Serialized context with disabled preemption */

	write_seqcount_begin(&foo_seqcount);

	/* ... [[write-side critical section]] ... */

	write_seqcount_end(&foo_seqcount);

Read path::

	do {
		seq = read_seqcount_begin(&foo_seqcount);

		/* ... [[read-side critical section]] ... */

	} while (read_seqcount_retry(&foo_seqcount, seq));


.. _seqlock_t:

Sequential locks (``seqlock_t``)
================================

This contains the :ref:`seqcount_t` mechanism 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 disable interrupts or bottom
halves respectively.

Initialization::

	/* 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::

	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::

	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::

	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::

	/* 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