ucas(9): Membar audit.
- Omit needless membar_enter before ipi_trigger_broadcast. This was
presumably intended to imply a happens-before relation for the
following two CPUs:
/* CPU doing ucas */
ucas_critical_enter()
ucas_critical_pausing_cpus = ncpu - 1 (A)
ipi_trigger_broadcast()
/* other CPU walking by whistling innocently */
IPI handler
ucas_critical_cpu_gate()
load ucas_critical_pausing_cpus (B)
That is, this was presumably meant to ensure (A) happens-before (B).
This relation is already guaranteed by ipi(9), so there is no need
for any explicit memory barrier.
- Issue a store-release in ucas_critical_cpu_gate so we have the
following happens-before relation which was otherwise not guaranteed
except if __HAVE_ATOMIC_AS_MEMBAR:
/* other CPU walking by whistling innocently */
...other logic touching the target ucas word... (A)
IPI handler
ucas_critical_cpu_gate()
...
atomic_dec_uint(&ucas_critical_pausing_cpus)
happens-before
/* CPU doing ucas */
ucas_critical_enter() -> ucas_critical_wait();
...touching the word with ufetch/ustore... (B)
We need to ensure the logic (A) on another CPU touching the target
ucas word happens-before we actually do the ucas at (B).
(a) This requires the other CPU to do a store-release on
ucas_critical_pausing_cpus in ucas_critical_cpu_gate, and
(b) this requires the ucas CPU to do a load-acquire on
ucas_critical_pausing_cpus in ucas_critical_wait.
Without _both_ sides -- store-release and then load-acquire -- there
is no such happens-before guarantee; another CPU may have a buffered
store, for instance, that clobbers the ucas.
For now, do the store-release with membar_exit conditional on
__HAVE_ATOMIC_AS_MEMBAR and then atomic_dec_uint -- later with the
C11 API we can dispense with the #ifdef and just use
atomic_fetch_add_explicit(..., memory_order_release). The
load-acquire we can do with atomic_load_acquire.
- Issue a load-acquire in ucas_critical_cpu_gate so we have the
following happens-before relation which was otherwise not guaranteed:
/* CPU doing ucas */
...ufetch/ustore... (A)
ucas_critical_exit()
ucas_critical_pausing_cpus = -1;
/* other CPU walking by whistling innocently */
IPI handler
ucas_critical_cpu_gate()
...
while (ucas_critical_pausing_cpus != -1)
spin;
...other logic touching the target ucas word... (B)
We need to ensure the logic (A) to do the ucas happens-before logic
that might use it on another CPU at (B).
(a) This requires that the ucas CPU do a store-release on
ucas_critical_pausing_cpus in ucas_critical_exit, and
(b) this requires that the other CPU do a load-acquire on
ucas_critical_pausing_cpus in ucas_critical_cpu_gate.
Without _both_ sides -- store-release and then load-acquire -- there
is no such happens-before guarantee; the other CPU might witness a
cached stale value of the target location but a new value of some
other location in the wrong order.
- Use atomic_load/store_* to avoid the appearance of races, e.g. for
sanitizers.
- Document which barriers pair up with which barriers and what they're
doing.
(riastradh)
diff -r1.14 -r1.15 src/sys/kern/subr_copy.c--- src/sys/kern/subr_copy.c 2020/05/23 23:42:43 1.14
+++ src/sys/kern/subr_copy.c 2022/02/11 17:53:28 1.15
@@ -1,4 +1,4 @@
-/* $NetBSD: subr_copy.c,v 1.14 2020/05/23 23:42:43 ad Exp $ */
+/* $NetBSD: subr_copy.c,v 1.15 2022/02/11 17:53:28 riastradh Exp $ */
/*-
* Copyright (c) 1997, 1998, 1999, 2002, 2007, 2008, 2019
@@ -80,7 +80,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_copy.c,v 1.14 2020/05/23 23:42:43 ad Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_copy.c,v 1.15 2022/02/11 17:53:28 riastradh Exp $");
#define __UFETCHSTORE_PRIVATE
#define __UCAS_PRIVATE
@@ -400,9 +400,32 @@
{
int count = SPINLOCK_BACKOFF_MIN;
- KASSERT(ucas_critical_pausing_cpus > 0);
+ KASSERT(atomic_load_relaxed(&ucas_critical_pausing_cpus) > 0);
+
+ /*
+ * Notify ucas_critical_wait that we have stopped. Using
+ * store-release ensures all our memory operations up to the
+ * IPI happen before the ucas -- no buffered stores on our end
+ * can clobber it later on, for instance.
+ *
+ * Matches atomic_load_acquire in ucas_critical_wait -- turns
+ * the following atomic_dec_uint into a store-release.
+ */
+#ifndef __HAVE_ATOMIC_AS_MEMBAR
+ membar_exit();
+#endif
atomic_dec_uint(&ucas_critical_pausing_cpus);
- while (ucas_critical_pausing_cpus != (u_int)-1) {
+
+ /*
+ * Wait for ucas_critical_exit to reopen the gate and let us
+ * proceed. Using a load-acquire ensures the ucas happens
+ * before any of our memory operations when we return from the
+ * IPI and proceed -- we won't observe any stale cached value
+ * that the ucas overwrote, for instance.
+ *
+ * Matches atomic_store_release in ucas_critical_exit.
+ */
+ while (atomic_load_acquire(&ucas_critical_pausing_cpus) != (u_int)-1) {
SPINLOCK_BACKOFF(count);
}
}
@@ -410,6 +433,7 @@
static int
ucas_critical_init(void)
{
+
ucas_critical_ipi = ipi_register(ucas_critical_cpu_gate, NULL);
return 0;
}
@@ -419,7 +443,16 @@
{
int count = SPINLOCK_BACKOFF_MIN;
- while (ucas_critical_pausing_cpus > 0) {
+ /*
+ * Wait for all CPUs to stop at the gate. Using a load-acquire
+ * ensures all memory operations before they stop at the gate
+ * happen before the ucas -- no buffered stores in other CPUs
+ * can clobber it later on, for instance.
+ *
+ * Matches membar_exit/atomic_dec_uint (store-release) in
+ * ucas_critical_cpu_gate.
+ */
+ while (atomic_load_acquire(&ucas_critical_pausing_cpus) > 0) {
SPINLOCK_BACKOFF(count);
}
}
@@ -444,8 +477,6 @@
mutex_enter(&cpu_lock);
ucas_critical_splcookie = splhigh();
ucas_critical_pausing_cpus = ncpu - 1;
- membar_enter();
-
ipi_trigger_broadcast(ucas_critical_ipi, true);
ucas_critical_wait();
return;
@@ -461,8 +492,17 @@
#if !defined(__HAVE_UCAS_MP) && defined(MULTIPROCESSOR)
if (ncpu > 1) {
- membar_exit();
- ucas_critical_pausing_cpus = (u_int)-1;
+ /*
+ * Open the gate and notify all CPUs in
+ * ucas_critical_cpu_gate that they can now proceed.
+ * Using a store-release ensures the ucas happens
+ * before any memory operations they issue after the
+ * IPI -- they won't observe any stale cache of the
+ * target word, for instance.
+ *
+ * Matches atomic_load_acquire in ucas_critical_cpu_gate.
+ */
+ atomic_store_release(&ucas_critical_pausing_cpus, (u_int)-1);
splx(ucas_critical_splcookie);
mutex_exit(&cpu_lock);
return;