NAME
pserialize —
passive serialization
mechanism
SYNOPSIS
#include <sys/pserialize.h>
pserialize_t
pserialize_create(
void);
void
pserialize_destroy(
pserialize_t
psz);
int
pserialize_read_enter(
void);
void
pserialize_read_exit(
int
s);
void
pserialize_perform(
pserialize_t
psz);
DESCRIPTION
Passive serialization is a reader / writer synchronisation mechanism designed
for lock-less read operations. The read operations may happen from software
interrupt at
IPL_SOFTCLOCK
.
FUNCTIONS
-
-
- pserialize_create()
- Allocate a new synchronisation object.
-
-
- pserialize_destroy()
- Destroy the synchronisation object. No synchronisation
activity should happen at this point.
-
-
- pserialize_read_enter()
- Enter the critical path of the reader side. Returns an IPL
value, which must be passed to
pserialize_read_exit(9).
Protected code path is not allowed to block.
-
-
- pserialize_read_exit()
- Exit the critical path of the reader side. Takes the IPL
value returned by
pserialize_read_enter(9).
-
-
- pserialize_perform()
- Perform the passive serialization on the writer side.
Passing of this function ensures that no readers are in action. Writers
must be additionally serialized with a separate mechanism, e.g.
mutex(9). Operation blocks
and it may only be performed from thread context.
EXAMPLES
Given a global database of frotz records:
struct frotz {
...
struct frotz *f_next;
};
static struct {
kmutex_t lock;
pserialize_t psz;
struct frotz *first;
} frobbotzim __cacheline_aligned;
Create a frotz and publish it, as a writer:
struct frotz *f = pool_get(&frotz_pool, PR_WAITOK);
/* Initialize f. */
...
mutex_enter(&frobbotzim.lock);
f->f_next = frobbotzim.first;
/*
* Publish the contents of f->f_next before we publish the
* pointer to f in frobbotzim.first.
*/
membar_producer();
frobbotzim.first = f;
mutex_exit(&frobbotzim.lock);
Find a frotz, as a reader:
struct frotz *f;
int error = ENOENT;
int s;
s = pserialize_read_enter();
for (f = frobbotzim.first; f != NULL; f = f->f_next) {
/* Fetch f before we fetch anything f points to. */
membar_datadep_consumer();
if (f->f_... == key) {
/*
* Grab whatever part of the frotz we need.
* Note that we can't use the frotz after
* pserialize_read_exit, without a stronger
* kind of reference, say a reference count
* managed by atomic_ops(3).
*/
*resultp = f->f_...;
error = 0;
break;
}
}
pserialize_read_exit(s);
return error;
Remove a frotz, as a writer, and free it once there are no more readers:
struct frotz **fp, *f;
mutex_enter(&frobbotzim.lock);
for (fp = &frobbotzim.first; (f = *fp) != NULL; fp = &f->f_next) {
if (f->f_... == key) {
/*
* Unhook it from the list. Readers may still
* be traversing the list at this point, so
* the next pointer must remain valid and
* memory must remain allocated.
*/
*fp = f->f_next;
break;
}
}
/*
* Wait for all existing readers to complete. New readers will
* not see f because the list no longer points to it.
*/
pserialize_perform(frobbotzim.psz);
/* Now nobody else can be touching f, so it is safe to free. */
mutex_exit(&frobbotzim.lock);
if (f != NULL)
pool_put(&frotz_pool, f);
CODE REFERENCES
The
pserialize is implemented within the file
sys/kern/subr_pserialize.c.
SEE ALSO
membar_ops(3),
condvar(9),
mutex(9),
rwlock(9)
Hennessy, et al.,
Passive serialization in a multitasking environment,
US Patent and Trademark Office, US Patent
4809168, February 28, 1989.
HISTORY
Passive serialization mechanism first appeared in
NetBSD
6.0.