/*
* Copyright (C) 2001-2004 Jakub Jermar
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* - Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* - Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* - The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/**
* @file rwlock.c
* @brief Reader/Writer locks.
*
* A reader/writer lock can be held by multiple readers at a time.
* Or it can be exclusively held by a sole writer at a time.
*
* These locks are not recursive.
* Because technique called direct hand-off is used, neither readers
* nor writers will suffer starvation.
*
* If there is a writer followed by a reader waiting for the rwlock
* and the writer times out, all leading readers are automatically woken up
* and allowed in.
*/
/*
* NOTE ON rwlock_holder_type
* This field is set on an attempt to acquire the exclusive mutex
* to the respective value depending whether the caller is a reader
* or a writer. The field is examined only if the thread had been
* previously blocked on the exclusive mutex. Thus it is save
* to store the rwlock type in the thread structure, because
* each thread can block on only one rwlock at a time.
*/
#include <synch/rwlock.h>
#include <synch/spinlock.h>
#include <synch/mutex.h>
#include <synch/waitq.h>
#include <synch/synch.h>
#include <adt/list.h>
#include <typedefs.h>
#include <arch/asm.h>
#include <arch.h>
#include <proc/thread.h>
#include <panic.h>
#define ALLOW_ALL 0
#define ALLOW_READERS_ONLY 1
static void let_others_in(rwlock_t *rwl, int readers_only);
static void release_spinlock(void *arg);
/** Initialize reader/writer lock
*
* Initialize reader/writer lock.
*
* @param rwl Reader/Writer lock.
*/
void rwlock_initialize(rwlock_t *rwl) {
spinlock_initialize(&rwl->lock, "rwlock_t");
mutex_initialize(&rwl->exclusive);
rwl->readers_in = 0;
}
/** Acquire reader/writer lock for reading
*
* Acquire reader/writer lock for reading.
* Timeout and willingness to block may be specified.
*
* @param rwl Reader/Writer lock.
* @param usec Timeout in microseconds.
* @param flags Specify mode of operation.
*
* For exact description of possible combinations of
* usec and flags, see comment for waitq_sleep_timeout().
*
* @return See comment for waitq_sleep_timeout().
*/
int _rwlock_write_lock_timeout(rwlock_t *rwl, __u32 usec, int flags)
{
ipl_t ipl;
int rc;
ipl = interrupts_disable();
spinlock_lock(&THREAD->lock);
THREAD->rwlock_holder_type = RWLOCK_WRITER;
spinlock_unlock(&THREAD->lock);
interrupts_restore(ipl);
/*
* Writers take the easy part.
* They just need to acquire the exclusive mutex.
*/
rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
if (SYNCH_FAILED(rc)) {
/*
* Lock operation timed out or was interrupted.
* The state of rwl is UNKNOWN at this point.
* No claims about its holder can be made.
*/
ipl = interrupts_disable();
spinlock_lock(&rwl->lock);
/*
* Now when rwl is locked, we can inspect it again.
* If it is held by some readers already, we can let
* readers from the head of the wait queue in.
*/
if (rwl->readers_in)
let_others_in(rwl, ALLOW_READERS_ONLY);
spinlock_unlock(&rwl->lock);
interrupts_restore(ipl);
}
return rc;
}
/** Acquire reader/writer lock for writing
*
* Acquire reader/writer lock for writing.
* Timeout and willingness to block may be specified.
*
* @param rwl Reader/Writer lock.
* @param usec Timeout in microseconds.
* @param flags Select mode of operation.
*
* For exact description of possible combinations of
* usec and flags, see comment for waitq_sleep_timeout().
*
* @return See comment for waitq_sleep_timeout().
*/
int _rwlock_read_lock_timeout(rwlock_t *rwl, __u32 usec, int flags)
{
int rc;
ipl_t ipl;
ipl = interrupts_disable();
spinlock_lock(&THREAD->lock);
THREAD->rwlock_holder_type = RWLOCK_READER;
spinlock_unlock(&THREAD->lock);
spinlock_lock(&rwl->lock);
/*
* Find out whether we can get what we want without blocking.
*/
rc = mutex_trylock(&rwl->exclusive);
if (SYNCH_FAILED(rc)) {
/*
* 'exclusive' mutex is being held by someone else.
* If the holder is a reader and there is no one
* else waiting for it, we can enter the critical
* section.
*/
if (rwl->readers_in) {
spinlock_lock(&rwl->exclusive.sem.wq.lock);
if (list_empty(&rwl->exclusive.sem.wq.head)) {
/*
* We can enter.
*/
spinlock_unlock(&rwl->exclusive.sem.wq.lock);
goto shortcut;
}
spinlock_unlock(&rwl->exclusive.sem.wq.lock);
}
/*
* In order to prevent a race condition when a reader
* could block another reader at the head of the waitq,
* we register a function to unlock rwl->lock
* after this thread is put asleep.
*/
#ifdef CONFIG_SMP
thread_register_call_me(release_spinlock, &rwl->lock);
#else
thread_register_call_me(release_spinlock, NULL);
#endif
rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
switch (rc) {
case ESYNCH_WOULD_BLOCK:
/*
* release_spinlock() wasn't called
*/
thread_register_call_me(NULL, NULL);
spinlock_unlock(&rwl->lock);
case ESYNCH_TIMEOUT:
case ESYNCH_INTERRUPTED:
/*
* The sleep timed out.
* We just restore interrupt priority level.
*/
case ESYNCH_OK_BLOCKED:
/*
* We were woken with rwl->readers_in already incremented.
* Note that this arrangement avoids race condition between
* two concurrent readers. (Race is avoided if 'exclusive' is
* locked at the same time as 'readers_in' is incremented.
* Same time means both events happen atomically when
* rwl->lock is held.)
*/
interrupts_restore(ipl);
break;
case ESYNCH_OK_ATOMIC:
panic("_mutex_lock_timeout()==ESYNCH_OK_ATOMIC\n");
break;
default:
panic("invalid ESYNCH\n");
break;
}
return rc;
}
shortcut:
/*
* We can increment readers_in only if we didn't go to sleep.
* For sleepers, rwlock_let_others_in() will do the job.
*/
rwl->readers_in++;
spinlock_unlock(&rwl->lock);
interrupts_restore(ipl);
return ESYNCH_OK_ATOMIC;
}
/** Release reader/writer lock held by writer
*
* Release reader/writer lock held by writer.
* Handoff reader/writer lock ownership directly
* to waiting readers or a writer.
*
* @param rwl Reader/Writer lock.
*/
void rwlock_write_unlock(rwlock_t *rwl)
{
ipl_t ipl;
ipl = interrupts_disable();
spinlock_lock(&rwl->lock);
let_others_in(rwl, ALLOW_ALL);
spinlock_unlock(&rwl->lock);
interrupts_restore(ipl);
}
/** Release reader/writer lock held by reader
*
* Release reader/writer lock held by reader.
* Handoff reader/writer lock ownership directly
* to a waiting writer or don't do anything if more
* readers poses the lock.
*
* @param rwl Reader/Writer lock.
*/
void rwlock_read_unlock(rwlock_t *rwl)
{
ipl_t ipl;
ipl = interrupts_disable();
spinlock_lock(&rwl->lock);
if (!--rwl->readers_in)
let_others_in(rwl, ALLOW_ALL);
spinlock_unlock(&rwl->lock);
interrupts_restore(ipl);
}
/** Direct handoff of reader/writer lock ownership.
*
* Direct handoff of reader/writer lock ownership
* to waiting readers or a writer.
*
* Must be called with rwl->lock locked.
* Must be called with interrupts_disable()'d.
*
* @param rwl Reader/Writer lock.
* @param readers_only See the description below.
*
* If readers_only is false: (unlock scenario)
* Let the first sleeper on 'exclusive' mutex in, no matter
* whether it is a reader or a writer. If there are more leading
* readers in line, let each of them in.
*
* Otherwise: (timeout scenario)
* Let all leading readers in.
*/
void let_others_in(rwlock_t *rwl, int readers_only)
{
rwlock_type_t type = RWLOCK_NONE;
thread_t *t = NULL;
bool one_more = true;
spinlock_lock(&rwl->exclusive.sem.wq.lock);
if (!list_empty(&rwl->exclusive.sem.wq.head))
t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
do {
if (t) {
spinlock_lock(&t->lock);
type = t->rwlock_holder_type;
spinlock_unlock(&t->lock);
}
/*
* If readers_only is true, we wake all leading readers
* if and only if rwl is locked by another reader.
* Assumption: readers_only ==> rwl->readers_in
*/
if (readers_only && (type != RWLOCK_READER))
break;
if (type == RWLOCK_READER) {
/*
* Waking up a reader.
* We are responsible for incrementing rwl->readers_in for it.
*/
rwl->readers_in++;
}
/*
* Only the last iteration through this loop can increment
* rwl->exclusive.sem.wq.missed_wakeup's. All preceeding
* iterations will wake up a thread.
*/
/* We call the internal version of waitq_wakeup, which
* relies on the fact that the waitq is already locked.
*/
_waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST);
t = NULL;
if (!list_empty(&rwl->exclusive.sem.wq.head)) {
t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
if (t) {
spinlock_lock(&t->lock);
if (t->rwlock_holder_type != RWLOCK_READER)
one_more = false;
spinlock_unlock(&t->lock);
}
}
} while ((type == RWLOCK_READER) && t && one_more);
spinlock_unlock(&rwl->exclusive.sem.wq.lock);
}
/** Release spinlock callback
*
* This is a callback function invoked from the scheduler.
* The callback is registered in _rwlock_read_lock_timeout().
*
* @param arg Spinlock.
*/
void release_spinlock(void *arg)
{
spinlock_unlock((spinlock_t *) arg);
}