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Ignore whitespace Rev 2541 → Rev 2682

/trunk/uspace/lib/libc/generic/futex.c
1,5 → 1,5
/*
* Copyright (c) 2006 Jakub Jermar
* Copyright (c) 2008 Jakub Jermar
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
41,8 → 41,8
 
/*
* 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:
* 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)
*
54,71 → 54,83
* 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
* 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
* 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
* 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
* 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.
* 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.
*
* 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.
* @param futex Futex.
* @param val Initialization value.
*/
void futex_initialize(atomic_t *futex, int val)
void futex_initialize(futex_t *futex, int val)
{
atomic_set(futex, val);
}
 
int futex_down(atomic_t *futex)
int futex_down(futex_t *futex)
{
return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
}
 
int futex_trydown(atomic_t *futex)
int futex_trydown(futex_t *futex)
{
return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
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().
* @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).
* @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 futex_down_timeout(futex_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);
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.
* 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;
130,9 → 142,10
 
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.
* 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;
141,7 → 154,8
case ESYNCH_OK_BLOCKED:
/*
* Enter the critical section.
* The futex counter has already been incremented for us.
* The futex counter has already been incremented for
* us.
*/
return ESYNCH_OK_BLOCKED;
break;
158,11 → 172,12
 
/** Up the futex.
*
* @param futex Futex.
* @param futex Futex.
*
* @return ENOENT if there is no such virtual address. Otherwise zero.
* @return ENOENT if there is no such virtual address. Otherwise
* zero.
*/
int futex_up(atomic_t *futex)
int futex_up(futex_t *futex)
{
long val;