Module Name: src
Committed By: thorpej
Date: Thu Aug 5 23:23:50 UTC 2021
Modified Files:
src/sys/kern [thorpej-futex2]: sys_futex.c
Log Message:
At the end of futex_wait(), when sleepq_block() returns 0, we would like
to assert that l->l_futex == NULL, because all of the code paths that awaken
a blocked thread in sys_futex.c itself clear l->l_futex. Unfortunately,
there are certain received-a-signal situations (e.g. SIGKILL) where
sleepq_block() will not return an error after being awakened by the signal,
rendering this assertion too strong.
So, rather than going down the rabbit hole of reasoning out and altering
long-standing behavior of the signals code, just don't assert there and
treat a zero-return from sleepq_block() as an aborted futex wait if
l->l_futex != NULL.
(Thanks chs@ for helping chase this one down.)
To generate a diff of this commit:
cvs rdiff -u -r1.12.4.1 -r1.12.4.2 src/sys/kern/sys_futex.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:1.12.4.1 src/sys/kern/sys_futex.c:1.12.4.2
--- src/sys/kern/sys_futex.c:1.12.4.1 Thu Aug 5 23:14:21 2021
+++ src/sys/kern/sys_futex.c Thu Aug 5 23:23:50 2021
@@ -1,4 +1,4 @@
-/* $NetBSD: sys_futex.c,v 1.12.4.1 2021/08/05 23:14:21 thorpej Exp $ */
+/* $NetBSD: sys_futex.c,v 1.12.4.2 2021/08/05 23:23:50 thorpej 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 1.12.4.1 2021/08/05 23:14:21 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: sys_futex.c,v 1.12.4.2 2021/08/05 23:23:50 thorpej Exp $");
/*
* Futexes
@@ -1122,8 +1122,23 @@ futex_wait(struct futex *f, int q, const
else if (error != ETIMEDOUT)
error = EINTR;
} else {
- KASSERT(l->l_futex == NULL);
- SDT_PROBE0(futex, wait, finish, normally);
+ f = l->l_futex;
+ if (__predict_false(f != NULL)) {
+ /*
+ * There are certain situations that can cause
+ * sleepq_block() to return 0 even if we were
+ * signalled (by e.g. SIGKILL). In this case,
+ * we will need to release our futex hold and
+ * return EINTR (we're probably about to die
+ * anyway).
+ */
+ SDT_PROBE0(futex, wait, finish, aborted);
+ l->l_futex = NULL;
+ futex_rele(f);
+ error = EINTR;
+ } else {
+ SDT_PROBE0(futex, wait, finish, normally);
+ }
}
return error;
