| 30,7 → 30,50 |
|---|
| #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); |
| 38,15 → 81,81 |
|---|
| |
| int futex_down(atomic_t *futex) |
| { |
| long val; |
| return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_BLOCKING); |
| } |
| |
| int futex_trydown(atomic_t *futex) |
| { |
| return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_NON_BLOCKING); |
| } |
| |
| /** Try to down the futex. |
| * |
| * @param futex Futex. |
| * @param usec Microseconds to wait. Zero value means sleep without timeout. |
| * @param trydown If usec is zero and trydown is non-zero, only conditional |
| * |
| * @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 trydown) |
| { |
| int rc; |
| |
| val = atomic_predec(futex); |
| if (val < 0) |
| return __SYSCALL1(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count); |
| while (atomic_predec(futex) < 0) { |
| rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) trydown); |
| |
| return 0; |
| 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 or futex. Otherwise zero. |
| */ |
| int futex_up(atomic_t *futex) |
| { |
| long val; |