| 0,0 → 1,192 |
|---|
| /* |
| * Copyright (c) 2008 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 <sys/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(futex_t *futex, int val) |
| { |
| atomic_set(futex, val); |
| } |
| |
| int futex_down(futex_t *futex) |
| { |
| return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE); |
| } |
| |
| int futex_trydown(futex_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(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); |
| |
| 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(futex_t *futex) |
| { |
| long val; |
| |
| val = atomic_postinc(futex); |
| if (val < 0) |
| return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count); |
| |
| return 0; |
| } |
| |
| /** @} |
| */ |