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Avoid mixing atomic futex changes and QAtomic

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Either the mix of futex and atomic, or the mix of 32-bit futex and
64-bit atomic doesn't work. In any case, the existing code leads to
bad behavior.

Fixes: QTBUG-92188
Change-Id: Icc6ba28d6e2465c373d00e84f4da2b92c037e797
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 2d9cc639a4a7a5e97979a6034364bd67dfa10c23)
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
This commit is contained in:
Allan Sandfeld Jensen 2021-05-14 10:43:11 +02:00
parent 562187fc55
commit 7b3635e013
1 changed files with 17 additions and 29 deletions
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44
src/corelib/thread/qsemaphore.cpp
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@ -358,47 +358,31 @@ void QSemaphore::release(int n)
quintptr prevValue = u.fetchAndAddRelease(nn); quintptr prevValue = u.fetchAndAddRelease(nn);
if (futexNeedsWake(prevValue)) { if (futexNeedsWake(prevValue)) {
#ifdef FUTEX_OP #ifdef FUTEX_OP
if (!futexHasWaiterCount) { if (futexHasWaiterCount) {
/*
On 32-bit systems, all waiters are waiting on the same address,
so we'll wake them all and ask the kernel to clear the high bit.
atomic {
int oldval = u;
u = oldval & ~(1 << 31);
futexWake(u, INT_MAX);
if (oldval == 0) // impossible condition
futexWake(u, INT_MAX);
}
*/
quint32 op = FUTEX_OP_ANDN | FUTEX_OP_OPARG_SHIFT;
quint32 oparg = 31;
quint32 cmp = FUTEX_OP_CMP_EQ;
quint32 cmparg = 0;
futexWakeOp(u, INT_MAX, INT_MAX, u, FUTEX_OP(op, oparg, cmp, cmparg));
} else {
/* /*
On 64-bit systems, the single-token waiters wait on the low half On 64-bit systems, the single-token waiters wait on the low half
and the multi-token waiters wait on the upper half. So we ask and the multi-token waiters wait on the upper half. So we ask
the kernel to wake up n single-token waiters and all multi-token the kernel to wake up n single-token waiters and all multi-token
waiters (if any), then clear the multi-token wait bit. waiters (if any), and clear the multi-token wait bit.
atomic { atomic {
int oldval = *upper; int oldval = *upper;
*upper = oldval & ~(1 << 31); *upper = oldval | 0;
futexWake(lower, n); futexWake(lower, n);
if (oldval < 0) // sign bit set if (oldval != 0) // always true
futexWake(upper, INT_MAX); futexWake(upper, INT_MAX);
} }
*/ */
quint32 op = FUTEX_OP_ANDN | FUTEX_OP_OPARG_SHIFT; quint32 op = FUTEX_OP_OR;
quint32 oparg = 31; quint32 oparg = 0;
quint32 cmp = FUTEX_OP_CMP_LT; quint32 cmp = FUTEX_OP_CMP_NE;
quint32 cmparg = 0; quint32 cmparg = 0;
u.fetchAndAndRelease(futexNeedsWakeAllBit - 1);
futexWakeOp(*futexLow32(&u), n, INT_MAX, *futexHigh32(&u), FUTEX_OP(op, oparg, cmp, cmparg)); futexWakeOp(*futexLow32(&u), n, INT_MAX, *futexHigh32(&u), FUTEX_OP(op, oparg, cmp, cmparg));
return;
} }
#else #endif
// Unset the bit and wake everyone. There are two possibibilies // Unset the bit and wake everyone. There are two possibilities
// under which a thread can set the bit between the AND and the // under which a thread can set the bit between the AND and the
// futexWake: // futexWake:
// 1) it did see the new counter value, but it wasn't enough for // 1) it did see the new counter value, but it wasn't enough for
@ -406,8 +390,12 @@ void QSemaphore::release(int n)
// 2) it did not see the new counter value, in which case its // 2) it did not see the new counter value, in which case its
// futexWait will fail. // futexWait will fail.
u.fetchAndAndRelease(futexNeedsWakeAllBit - 1); u.fetchAndAndRelease(futexNeedsWakeAllBit - 1);
if (futexHasWaiterCount) {
futexWakeAll(*futexLow32(&u));
futexWakeAll(*futexHigh32(&u));
} else {
futexWakeAll(u); futexWakeAll(u);
#endif }
} }
return; return;
} }