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.Dd January 26, 2016 .Dt PSERIALIZE 9 .Os .Sh NAME .Nm pserialize .Nd passive serialization mechanism .Sh SYNOPSIS n sys/pserialize.h .Ft pserialize_t .Fn pserialize_create "void" .Ft void .Fn pserialize_destroy "pserialize_t psz" .Ft int .Fn pserialize_read_enter "void" .Ft void .Fn pserialize_read_exit "int s" .Ft void .Fn pserialize_perform "pserialize_t psz" -----
.Sh DESCRIPTION Passive serialization is a reader / writer synchronisation mechanism designed for lock-less read operations. The read operations may happen from software interrupt at .Dv IPL_SOFTCLOCK . .Sh FUNCTIONS l -tag -width compact t Fn pserialize_create Allocate a new synchronisation object. t Fn pserialize_destroy Destroy the synchronisation object. No synchronisation activity should happen at this point. t Fn pserialize_read_enter Enter the critical path of the reader side. Returns an IPL value, which must be passed to .Xr pserialize_read_exit 9 . Protected code path is not allowed to block. t Fn pserialize_read_exit Exit the critical path of the reader side. Takes the IPL value returned by .Xr pserialize_read_enter 9 . t Fn 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. .Xr mutex 9 . Operation blocks and it may only be performed from thread context. .El -----
.Sh EXAMPLES Given a global database of frotz records: d -literal struct frotz { ... struct frotz *f_next; }; static struct { kmutex_t lock; pserialize_t psz; struct frotz *first; } frobbotzim __cacheline_aligned; .Ed
p Create a frotz and publish it, as a writer: d -literal 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); .Ed
p Find a frotz, as a reader: d -literal 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; .Ed
p
Remove a frotz, as a writer, and free it once there are no more
readers:
d -literal 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);
.Ed
-----
.Sh CODE REFERENCES
The
.Nm
is implemented within the file
a sys/kern/subr_pserialize.c . .Sh SEE ALSO .Xr membar_ops 3 , .Xr condvar 9 , .Xr mutex 9 , .Xr rwlock 9 .Rs .%A Hennessy, et al. .%T "Passive serialization in a multitasking environment" .%I US Patent and Trademark Office .%D February 28, 1989 .%N US Patent 4809168 .Re .Sh HISTORY Passive serialization mechanism first appeared in .Nx 6.0 .