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.Dd July 29, 2019
.Dt SEQC 9
.Os
.Sh NAME
.Nm seqc_consistent ,
.Nm seqc_read ,
.Nm seqc_write_begin ,
.Nm seqc_write_end
.Nd "lockless read algorithm"
.Sh SYNOPSIS
.In sys/seqc.h
.Ft void
.Fn seqc_write_begin "seqc_t *seqcp"
.Ft void
.Fn seqc_write_end "seqc_t *seqcp"
.Ft seqc_t
.Fn seqc_read "seqc_t *seqcp"
.Ft seqc_t
.Fn seqc_consistent "const seqc_t *seqcp" "seqc_t oldseqc"
.Sh DESCRIPTION
The
.Nm seqc
allows zero or more readers and zero or one writer to concurrently access
an object, providing a consistent snapshot of the object for readers.
No mutual exclusion between readers and writers is required,
but readers may be starved indefinitely by writers.
.Pp
The functions
.Fn seqc_write_begin
and
.Fn seqc_write_end
are used to create a transaction for writer, and notify the readers that the
object will be modified.
.Pp
The
.Fn seqc_read
function returns the current sequence number.
If a writer has started a transaction, this function will spin until the
transaction has ended.
.Pp
The
.Fn seqc_consistent
function compares the sequence number with a previously fetched value.
The
.Fa oldseqc
variable should contain a sequence number from the beginning of read
transaction.
.Pp
The reader at the end of a transaction checks if the sequence number has
changed.
If the sequence number didn't change the object wasn't modified, and fetched
variables are valid.
If the sequence number changed the object was modified and the fetch should be
repeated.
In case when sequence number is odd the object change is in progress and the
reader will wait until the write will the sequence number will become even.
.Sh EXAMPLES
The following example for a writer changees the
.Va var1
and
.Va var2
variables in the
.Va obj
structure:
.Bd -literal
lock_exclusive(&obj->lock);
seqc_write_begin(&obj->seqc);
obj->var1 = 1;
obj->var2 = 2;
seqc_write_end(&obj->seqc);
unlock_exclusive(&obj->lock);
.Ed
.Pp
The following example for a reader reads the
.Va var1
and
.Va var2
variables from the
.Va obj
structure.
In the case where the sequence number was changed it restarts the whole process.
.Bd -literal
int var1, var2;
seqc_t seqc;

for (;;) {
	seqc = seqc_read(&obj->seqc);
	var1 = obj->var1;
	var2 = obj->var2;
	if (seqc_consistent(&obj->seqc, seqc))
		break;
}
.Ed
.Sh AUTHORS
The
.Nm seqc
functions was implemented by
.An Mateusz Guzik Aq Mt mjg@FreeBSD.org .
This manual page was written by
.An Mariusz Zaborski Aq Mt oshogbo@FreeBSD.org .
.Sh CAVEATS
There is no guarantee of progress for readers.
In case when there are a lot of writers the reader can be starved.
This concern may be solved by returning error after a few attempts.
.Pp
Theoretically if reading takes a very long time, and when there are many writers
the counter may overflow and wrap around to the same value.
In that case the reader will not notice that the object was changed.
Given that this needs 4 billion transactional writes across a single contended
reader, it is unlikely to ever happen.
This could be avoided by extending the interface to allow 64-bit counters.