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); |
} |
|
val = atomic_predec(futex); |
if (val < 0) |
return __SYSCALL1(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count); |
int futex_trydown(atomic_t *futex) |
{ |
return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_NON_BLOCKING); |
} |
|
return 0; |
/** 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; |
|
while (atomic_predec(futex) < 0) { |
rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) trydown); |
|
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; |