Module Name: src
Committed By: riastradh
Date: Sat Jan 18 07:26:21 UTC 2025
Modified Files:
src/sys/kern: sys_futex.c
src/tests/lib/libc/sys: t_futex_ops.c
Log Message:
futex(2): Fix return value of FUTEX_CMP_REQUEUE.
The return value is the number of waiters woken _or requeued_, not
just the number of waiters woken:
FUTEX_CMP_REQUEUE
Returns the total number of waiters that were woken up or
requeued to the futex for the futex word at uaddr2. If
this value is greater than val, then the difference is the
number of waiters requeued to the futex for the futex word
at uaddr2.
https://man7.org/linux/man-pages/man2/futex.2.html
While here, clarify some of the arguments with comments so it's not
quite so cryptic with val/val2/val3 everywhere.
PR kern/56828: futex calls in Linux emulation sometimes hang
To generate a diff of this commit:
cvs rdiff -u -r1.21 -r1.22 src/sys/kern/sys_futex.c
cvs rdiff -u -r1.9 -r1.10 src/tests/lib/libc/sys/t_futex_ops.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/sys_futex.c
diff -u src/sys/kern/sys_futex.c:1.21 src/sys/kern/sys_futex.c:1.22
--- src/sys/kern/sys_futex.c:1.21 Sat Jan 18 07:26:06 2025
+++ src/sys/kern/sys_futex.c Sat Jan 18 07:26:21 2025
@@ -1,4 +1,4 @@
-/* $NetBSD: sys_futex.c,v 1.21 2025/01/18 07:26:06 riastradh Exp $ */
+/* $NetBSD: sys_futex.c,v 1.22 2025/01/18 07:26:21 riastradh Exp $ */
/*-
* Copyright (c) 2018, 2019, 2020 The NetBSD Foundation, Inc.
@@ -30,7 +30,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: sys_futex.c,v 1.21 2025/01/18 07:26:06 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: sys_futex.c,v 1.22 2025/01/18 07:26:21 riastradh Exp $");
/*
* Futexes
@@ -996,15 +996,15 @@ futex_wait(struct futex_wait *fw, const
*
* Wake up to nwake waiters on f matching bitset; then, if f2 is
* provided, move up to nrequeue remaining waiters on f matching
- * bitset to f2. Return the number of waiters actually woken.
- * Caller must hold the locks of f and f2, if provided.
+ * bitset to f2. Return the number of waiters actually woken or
+ * requeued. Caller must hold the locks of f and f2, if provided.
*/
static unsigned
futex_wake(struct futex *f, unsigned nwake, struct futex *f2,
unsigned nrequeue, int bitset)
{
struct futex_wait *fw, *fw_next;
- unsigned nwoken = 0;
+ unsigned nwoken_or_requeued = 0;
int hold_error __diagused;
KASSERT(mutex_owned(&f->fx_qlock));
@@ -1025,7 +1025,7 @@ futex_wake(struct futex *f, unsigned nwa
cv_broadcast(&fw->fw_cv);
mutex_exit(&fw->fw_lock);
nwake--;
- nwoken++;
+ nwoken_or_requeued++;
/*
* Drop the futex reference on behalf of the
* waiter. We assert this is not the last
@@ -1054,6 +1054,16 @@ futex_wake(struct futex *f, unsigned nwa
mutex_exit(&fw->fw_lock);
nrequeue--;
/*
+ * PR kern/59004: Missing constant for upper
+ * bound on systemwide number of lwps
+ */
+ KASSERT(nwoken_or_requeued <
+ MIN(PID_MAX*MAXMAXLWP, FUTEX_TID_MASK));
+ __CTASSERT(UINT_MAX >=
+ MIN(PID_MAX*MAXMAXLWP, FUTEX_TID_MASK));
+ if (++nwoken_or_requeued == 0) /* paranoia */
+ nwoken_or_requeued = UINT_MAX;
+ /*
* Transfer the reference from f to f2.
* As above, we assert that we are not
* dropping the last reference to f here.
@@ -1072,8 +1082,8 @@ futex_wake(struct futex *f, unsigned nwa
KASSERT(nrequeue == 0);
}
- /* Return the number of waiters woken. */
- return nwoken;
+ /* Return the number of waiters woken or requeued. */
+ return nwoken_or_requeued;
}
/*
@@ -1303,7 +1313,8 @@ futex_func_wake(bool shared, int *uaddr,
* number woken.
*/
futex_queue_lock(f);
- nwoken = futex_wake(f, val, NULL, 0, val3);
+ nwoken = futex_wake(f, /*nwake*/val, NULL, /*nrequeue*/0,
+ /*bitset*/val3);
futex_queue_unlock(f);
/* Release the futex. */
@@ -1327,7 +1338,7 @@ futex_func_requeue(bool shared, int op,
int val2, int val3, register_t *retval)
{
struct futex *f = NULL, *f2 = NULL;
- unsigned nwoken = 0; /* default to zero woken on early return */
+ unsigned nwoken_or_requeued = 0; /* default to zero on early return */
int error;
/* Reject negative number of wakeups or requeues. */
@@ -1371,13 +1382,15 @@ futex_func_requeue(bool shared, int op,
error = EAGAIN;
} else {
error = 0;
- nwoken = futex_wake(f, val, f2, val2, FUTEX_BITSET_MATCH_ANY);
+ nwoken_or_requeued = futex_wake(f, /*nwake*/val,
+ f2, /*nrequeue*/val2,
+ FUTEX_BITSET_MATCH_ANY);
}
futex_queue_unlock2(f, f2);
out:
- /* Return the number of waiters woken. */
- *retval = nwoken;
+ /* Return the number of waiters woken or requeued. */
+ *retval = nwoken_or_requeued;
/* Release the futexes if we got them. */
if (f2)
@@ -1563,10 +1576,16 @@ futex_func_wake_op(bool shared, int *uad
if (error)
goto out_unlock;
} while (actual != oldval);
- nwoken = (f ? futex_wake(f, val, NULL, 0, FUTEX_BITSET_MATCH_ANY) : 0);
- if (f2 && futex_compute_cmp(oldval, val3))
- nwoken += futex_wake(f2, val2, NULL, 0,
+ if (f == NULL) {
+ nwoken = 0;
+ } else {
+ nwoken = futex_wake(f, /*nwake*/val, NULL, /*nrequeue*/0,
+ FUTEX_BITSET_MATCH_ANY);
+ }
+ if (f2 && futex_compute_cmp(oldval, val3)) {
+ nwoken += futex_wake(f2, /*nwake*/val2, NULL, /*nrequeue*/0,
FUTEX_BITSET_MATCH_ANY);
+ }
/* Success! */
error = 0;
@@ -1855,8 +1874,10 @@ release_futex(uintptr_t const uptr, lwpi
*
* XXX eventual PI handling?
*/
- if (oldval & FUTEX_WAITERS)
- (void)futex_wake(f, 1, NULL, 0, FUTEX_BITSET_MATCH_ANY);
+ if (oldval & FUTEX_WAITERS) {
+ (void)futex_wake(f, /*nwake*/1, NULL, /*nrequeue*/0,
+ FUTEX_BITSET_MATCH_ANY);
+ }
/* Unlock the queue and release the futex. */
out: futex_queue_unlock(f);
Index: src/tests/lib/libc/sys/t_futex_ops.c
diff -u src/tests/lib/libc/sys/t_futex_ops.c:1.9 src/tests/lib/libc/sys/t_futex_ops.c:1.10
--- src/tests/lib/libc/sys/t_futex_ops.c:1.9 Sat Jan 18 07:26:06 2025
+++ src/tests/lib/libc/sys/t_futex_ops.c Sat Jan 18 07:26:21 2025
@@ -1,4 +1,4 @@
-/* $NetBSD: t_futex_ops.c,v 1.9 2025/01/18 07:26:06 riastradh Exp $ */
+/* $NetBSD: t_futex_ops.c,v 1.10 2025/01/18 07:26:21 riastradh Exp $ */
/*-
* Copyright (c) 2019, 2020 The NetBSD Foundation, Inc.
@@ -29,7 +29,7 @@
#include <sys/cdefs.h>
__COPYRIGHT("@(#) Copyright (c) 2019, 2020\
The NetBSD Foundation, inc. All rights reserved.");
-__RCSID("$NetBSD: t_futex_ops.c,v 1.9 2025/01/18 07:26:06 riastradh Exp $");
+__RCSID("$NetBSD: t_futex_ops.c,v 1.10 2025/01/18 07:26:21 riastradh Exp $");
#include <sys/fcntl.h>
#include <sys/mman.h>
@@ -825,11 +825,8 @@ do_futex_requeue_test(int flags, int op)
*/
/* Move all waiters from 0 to 1. */
- atf_tc_expect_fail("PR kern/56828:"
- " futex calls in Linux emulation sometimes hang");
ATF_CHECK(__futex(&futex_word, op | flags,
0, NULL, &futex_word1, INT_MAX, good_val3) == 4);
- atf_tc_expect_pass();
/*
* FUTEX 0: 0 LWPs
@@ -849,11 +846,8 @@ do_futex_requeue_test(int flags, int op)
*/
/* Wake one waiter on 1, move one waiter to 0. */
- atf_tc_expect_fail("PR kern/56828:"
- " futex calls in Linux emulation sometimes hang");
ATF_CHECK(__futex(&futex_word1, op | flags,
1, NULL, &futex_word, 1, good_val3) == 2);
- atf_tc_expect_pass();
/*
* FUTEX 0: 1 LWP
