>From 99b7e15342fda1591838efc068d2624457afd5b0 Mon Sep 17 00:00:00 2001
From: Akira Yokosawa <akiyks@xxxxxxxxx>
Date: Mon, 19 Nov 2018 23:02:09 +0900
Subject: [PATCH 2/3] locking: Get rid of ACCESS_ONCE()
Also add READ_ONCE() and WRITE_ONCE() to racy accesses in
xchglock.c.
Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx>
---
CodeSamples/locking/xchglock.c | 10 +++++-----
locking/locking.tex | 10 +++++-----
2 files changed, 10 insertions(+), 10 deletions(-)
diff --git a/CodeSamples/locking/xchglock.c b/CodeSamples/locking/xchglock.c
index 5877920..a5e3403 100644
--- a/CodeSamples/locking/xchglock.c
+++ b/CodeSamples/locking/xchglock.c
@@ -28,7 +28,7 @@ typedef int xchglock_t; //\lnlbl{typedef}
void xchg_lock(xchglock_t *xp) //\lnlbl{lock:b}
{
while (xchg(xp, 1) == 1) { //\lnlbl{lock:atmxchg}
- while (*xp == 1) //\lnlbl{lock:inner:b}
+ while (READ_ONCE(*xp) == 1) //\lnlbl{lock:inner:b}
continue; //\lnlbl{lock:inner:e}
}
} //\lnlbl{lock:e}
@@ -59,9 +59,9 @@ void *test_xchg_lock(void *arg)
run_on(me);
atomic_inc(&nthreadsrunning);
- while (ACCESS_ONCE(goflag) == GOFLAG_INIT)
+ while (READ_ONCE(goflag) == GOFLAG_INIT)
poll(NULL, 0, 1);
- while (ACCESS_ONCE(goflag) == GOFLAG_RUN) {
+ while (READ_ONCE(goflag) == GOFLAG_RUN) {
xchg_lock(&testlock);
if (owner != -1)
lockerr++;
@@ -86,11 +86,11 @@ int main(int argc, char *argv[])
smp_mb();
while (atomic_read(&nthreadsrunning) < nthreads)
poll(NULL, 0, 1);
- goflag = GOFLAG_RUN;
+ WRITE_ONCE(goflag, GOFLAG_RUN);
smp_mb();
poll(NULL, 0, 10000);
smp_mb();
- goflag = GOFLAG_STOP;
+ WRITE_ONCE(goflag, GOFLAG_STOP);
smp_mb();
wait_all_threads();
printf("lockacqs = %lu, lockerr = %lu\n", lockacqs, lockerr);
diff --git a/locking/locking.tex b/locking/locking.tex
index 92ed2f2..874fb3b 100644
--- a/locking/locking.tex
+++ b/locking/locking.tex
@@ -1463,7 +1463,7 @@ void force_quiescent_state(struct rcu_node *rnp_leaf)
struct rcu_node *rnp_old = NULL;
for (; rnp != NULL; rnp = rnp->parent) { \lnlbl[loop:b]
- ret = (ACCESS_ONCE(gp_flags)) || \lnlbl[flag_set]
+ ret = (READ_ONCE(gp_flags)) || \lnlbl[flag_set]
!raw_spin_trylock(&rnp->fqslock);\lnlbl[trylock]
if (rnp_old != NULL) \lnlbl[non_NULL]
raw_spin_unlock(&rnp_old->fqslock);\lnlbl[rel1]
@@ -1471,11 +1471,11 @@ void force_quiescent_state(struct rcu_node *rnp_leaf)
return; \lnlbl[return]
rnp_old = rnp; \lnlbl[save]
} \lnlbl[loop:e]
- if (!ACCESS_ONCE(gp_flags)) { \lnlbl[flag_not_set]
- ACCESS_ONCE(gp_flags) = 1; \lnlbl[set_flag]
+ if (!READ_ONCE(gp_flags)) { \lnlbl[flag_not_set]
+ WRITE_ONCE(gp_flags, 1); \lnlbl[set_flag]
do_force_quiescent_state(); \lnlbl[invoke]
schedule_timeout_interruptible(HZ / 10);\lnlbl[wait]
- ACCESS_ONCE(gp_flags) = 0; \lnlbl[clr_flag]
+ WRITE_ONCE(gp_flags, 0); \lnlbl[clr_flag]
}
raw_spin_unlock(&rnp_old->fqslock); \lnlbl[rel2]
}
@@ -1649,7 +1649,7 @@ the lock, thus marking it as having been released.
\QuickQuizAnswer{
This can be a legitimate implementation, but only if
this store is preceded by a memory barrier and makes use
- of \co{ACCESS_ONCE()}.
+ of \co{WRITE_ONCE()}.
The memory barrier is not required when the \co{xchg()}
operation is used because this operation implies a
full memory barrier due to the fact that it returns
--
2.7.4
