/*
* Acquire ticket lock. Increment the tail of the queue and use the original
* value as the ticket value. Wait until the head of the queue equals the
* ticket value. The futex used to wait depends on the ticket value in order
* to avoid that all threads get woken up every time a ticket lock is
* released. That last effect is sometimes called the "thundering herd"
* effect.
*
* See also Nick Piggin, x86: FIFO ticket spinlocks, Linux kernel mailing list
* (http://lkml.org/lkml/2007/11/1/125) for more info.
*/
static void acquire_sched_lock(struct sched_lock *p, ThreadId tid, SchedLockKind slk)
{
unsigned ticket, futex_value;
volatile unsigned *futex;
SysRes sres;
ticket = __sync_fetch_and_add(&p->tail, 1);
futex = &p->futex[ticket & TL_FUTEX_MASK];
if (s_debug)
VG_(printf)("[%d/%d] acquire: ticket %d\n", VG_(getpid)(),
VG_(gettid)(), ticket);
for (;;) {
futex_value = *futex;
__sync_synchronize();
if (ticket == p->head)
break;
if (s_debug)
VG_(printf)("[%d/%d] acquire: ticket %d - waiting until"
" futex[%ld] != %d\n", VG_(getpid)(),
VG_(gettid)(), ticket, (long)(futex - p->futex),
futex_value);
sres = VG_(do_syscall3)(__NR_futex, (UWord)futex,
VKI_FUTEX_WAIT | VKI_FUTEX_PRIVATE_FLAG,
futex_value);
if (sr_isError(sres) && sres._val != VKI_EAGAIN) {
VG_(printf)("futex_wait() returned error code %ld\n", sres._val);
vg_assert(False);
}
}
__sync_synchronize();
INNER_REQUEST(ANNOTATE_RWLOCK_ACQUIRED(p, /*is_w*/1));
vg_assert(p->owner == 0);
p->owner = VG_(gettid)();
}
/* get a token */
void ML_(sema_down)( vg_sema_t *sema, Bool as_LL )
{
HChar buf[2];
Int ret;
Int lwpid = VG_(gettid)();
vg_assert(sema->owner_lwpid != lwpid); /* can't have it already */
vg_assert(sema->pipe[0] != sema->pipe[1]);
again:
buf[0] = buf[1] = 0;
ret = VG_(read)(sema->pipe[0], buf, 1);
INNER_REQUEST(ANNOTATE_RWLOCK_ACQUIRED(sema, /*is_w*/1));
if (ret != 1)
VG_(debugLog)(0, "scheduler",
"VG_(sema_down): read returned %d\n", ret);
if (ret == -VKI_EINTR)
goto again;
vg_assert(ret == 1); /* should get exactly 1 token */
vg_assert(buf[0] >= 'A' && buf[0] <= 'Z');
vg_assert(buf[1] == 0);
if (sema_char == 'Z') sema_char = 'A'; else sema_char++;
sema->owner_lwpid = lwpid;
sema->held_as_LL = as_LL;
}
static void rwlock_wrlock(rwlock_t* p)
{
while (1)
{
while (__sync_val_compare_and_swap(&p->locked, 0, 1) == 1)
;
if (p->reader_count == 0)
break;
pthread_yield();
__sync_fetch_and_sub(&p->locked, 1);
}
p->writer_count++;
assert(p->reader_count >= 0);
assert(p->writer_count >= 0);
assert(p->reader_count == 0 || p->writer_count == 0);
__sync_fetch_and_sub(&p->locked, 1);
ANNOTATE_RWLOCK_ACQUIRED(p, 1);
}
