pserialize_read_exit(9) - NetBSD Manual Pages

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PSERIALIZE(9)          NetBSD Kernel Developer's Manual          PSERIALIZE(9)


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). Pro- tected 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. NetBSD 9.1 January 26, 2016 NetBSD 9.1
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