/*
* Copyright (c) 2006 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.
*/
/** @addtogroup libc
* @{
*/
/** @file
*/
#include <futex.h>
#include <atomic.h>
#include <libc.h>
#include <stdio.h>
#include <types.h>
#include <kernel/synch/synch.h>
/*
* Note about race conditions.
* Because of non-atomic nature of operations performed sequentially on the futex
* counter and the futex wait queue, there is a race condition:
*
* (wq->missed_wakeups == 1) && (futex->count = 1)
*
* Scenario 1 (wait queue timeout vs. futex_up()):
* 1. assume wq->missed_wakeups == 0 && futex->count == -1
* (ie. thread A sleeping, thread B in the critical section)
* 2. A receives timeout and gets removed from the wait queue
* 3. B wants to leave the critical section and calls futex_up()
* 4. B thus changes futex->count from -1 to 0
* 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
* 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
* 7. A fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
*
* Scenario 2 (conditional down operation vs. futex_up)
* 1. assume wq->missed_wakeups == 0 && futex->count == 0
* (i.e. thread A is in the critical section)
* 2. thread B performs futex_trydown() operation and changes futex->count from 0 to -1
* B is now obliged to call SYS_FUTEX_SLEEP syscall
* 3. A wants to leave the critical section and does futex_up()
* 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP syscall
* 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
* 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from 0 to 1
* 6. B fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
*
* Both scenarios allow two threads to be in the critical section simultaneously.
* One without kernel intervention and the other through wq->missed_wakeups being 1.
*
* To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
* in the wait queue, fixes the futex counter and RETRIES the whole operation again.
*
*/
/** Initialize futex counter.
*
* @param futex Futex.
* @param val Initialization value.
*/
void futex_initialize(atomic_t *futex, int val)
{
atomic_set(futex, val);
}
int futex_down(atomic_t *futex)
{
return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
}
int futex_trydown(atomic_t *futex)
{
return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
}
/** Try to down the futex.
*
* @param futex Futex.
* @param usec Microseconds to wait. Zero value means sleep without timeout.
* @param flags Select mode of operation. See comment for waitq_sleep_timeout().
*
* @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
* and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
* not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
* operation could not be carried out atomically (if requested so).
*/
int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
{
int rc;
while (atomic_predec(futex) < 0) {
rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
switch (rc) {
case ESYNCH_OK_ATOMIC:
/*
* Because of a race condition between timeout and futex_up()
* and between conditional futex_down_timeout() and futex_up(),
* we have to give up and try again in this special case.
*/
atomic_inc(futex);
break;
case ESYNCH_TIMEOUT:
atomic_inc(futex);
return ESYNCH_TIMEOUT;
break;
case ESYNCH_WOULD_BLOCK:
/*
* The conditional down operation should be implemented this way.
* The userspace-only variant tends to accumulate missed wakeups
* in the kernel futex wait queue.
*/
atomic_inc(futex);
return ESYNCH_WOULD_BLOCK;
break;
case ESYNCH_OK_BLOCKED:
/*
* Enter the critical section.
* The futex counter has already been incremented for us.
*/
return ESYNCH_OK_BLOCKED;
break;
default:
return rc;
}
}
/*
* Enter the critical section.
*/
return ESYNCH_OK_ATOMIC;
}
/** Up the futex.
*
* @param futex Futex.
*
* @return ENOENT if there is no such virtual address. Otherwise zero.
*/
int futex_up(atomic_t *futex)
{
long val;
val = atomic_postinc(futex);
if (val < 0)
return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
return 0;
}
/** @}
*/