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Date: Sat, 16 Apr 2016 10:34:38 -0700
From: Davidlohr Bueso <dave@...olabs.net>
To: mingo@...nel.org, akpm@...ux-foundation.org
Cc: dave@...olabs.net, linux-kernel@...r.kernel.org,
Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH -next] kernel: Replace ACCESS_ONCE with READ/WRITE_ONCE
We already have a number of calls converted, but still a decent
amount of ACCESS_ONCE callers in core-code, which is somewhat
troubling. Convert more users (even if some are scalar types,
we want to someday get rid of ACCESS_ONCE altogether I suppose.
Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
Documentation/atomic_ops.txt | 18 +++++++++---------
include/linux/bitops.h | 2 +-
include/linux/llist.h | 2 +-
kernel/acct.c | 4 ++--
kernel/events/core.c | 6 +++---
kernel/events/ring_buffer.c | 2 +-
kernel/exit.c | 2 +-
kernel/kthread.c | 2 +-
kernel/task_work.c | 6 +++---
kernel/trace/ring_buffer.c | 2 +-
kernel/trace/trace.h | 2 +-
kernel/trace/trace_stack.c | 2 +-
kernel/user_namespace.c | 2 +-
kernel/watchdog.c | 4 ++--
kernel/workqueue.c | 4 ++--
15 files changed, 30 insertions(+), 30 deletions(-)
diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index c9d1cacb4395..9e880d221820 100644
--- a/Documentation/atomic_ops.txt
+++ b/Documentation/atomic_ops.txt
@@ -90,10 +90,10 @@ compiler optimizes the section accessing atomic_t variables.
Properly aligned pointers, longs, ints, and chars (and unsigned
equivalents) may be atomically loaded from and stored to in the same
-sense as described for atomic_read() and atomic_set(). The ACCESS_ONCE()
-macro should be used to prevent the compiler from using optimizations
-that might otherwise optimize accesses out of existence on the one hand,
-or that might create unsolicited accesses on the other.
+sense as described for atomic_read() and atomic_set(). The READ_ONCE()
+and WRITE_ONCE() macros should be used to prevent the compiler from using
+optimizations that might otherwise optimize accesses out of existence on
+the one hand, or that might create unsolicited accesses on the other.
For example consider the following code:
@@ -112,7 +112,7 @@ the following:
If you don't want the compiler to do this (and you probably don't), then
you should use something like the following:
- while (ACCESS_ONCE(a) < 0)
+ while (READ_ONCE(a) < 0)
do_something();
Alternatively, you could place a barrier() call in the loop.
@@ -141,7 +141,7 @@ of registers: reloading from variable a could save a flush to the
stack and later reload. To prevent the compiler from attacking your
code in this manner, write the following:
- tmp_a = ACCESS_ONCE(a);
+ tmp_a = READ_ONCE(a);
do_something_with(tmp_a);
do_something_else_with(tmp_a);
@@ -166,14 +166,14 @@ that expected b to never have the value 42 if a was zero. To prevent
the compiler from doing this, write something like:
if (a)
- ACCESS_ONCE(b) = 9;
+ WRITE_ONCE(b, 9);
else
- ACCESS_ONCE(b) = 42;
+ WRITE_ONCE(b, 42);
Don't even -think- about doing this without proper use of memory barriers,
locks, or atomic operations if variable a can change at runtime!
-*** WARNING: ACCESS_ONCE() DOES NOT IMPLY A BARRIER! ***
+*** WARNING: NEITHER READ_ONCE() OR WRITE_ONCE() IMPLY A BARRIER! ***
Now, we move onto the atomic operation interfaces typically implemented with
the help of assembly code.
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index defeaac0745f..26da22da6f63 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -219,7 +219,7 @@ static inline unsigned long __ffs64(u64 word)
typeof(*ptr) old, new; \
\
do { \
- old = ACCESS_ONCE(*ptr); \
+ old = READ_ONCE(*ptr); \
new = (old & ~mask) | bits; \
} while (cmpxchg(ptr, old, new) != old); \
\
diff --git a/include/linux/llist.h b/include/linux/llist.h
index fd4ca0b4fe0f..2381ace84815 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -157,7 +157,7 @@ static inline void init_llist_head(struct llist_head *list)
*/
static inline bool llist_empty(const struct llist_head *head)
{
- return ACCESS_ONCE(head->first) == NULL;
+ return READ_ONCE(head->first) == NULL;
}
static inline struct llist_node *llist_next(struct llist_node *node)
diff --git a/kernel/acct.c b/kernel/acct.c
index 74963d192c5d..fca2d25c3708 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -144,7 +144,7 @@ static struct bsd_acct_struct *acct_get(struct pid_namespace *ns)
again:
smp_rmb();
rcu_read_lock();
- res = to_acct(ACCESS_ONCE(ns->bacct));
+ res = to_acct(READ_ONCE(ns->bacct));
if (!res) {
rcu_read_unlock();
return NULL;
@@ -156,7 +156,7 @@ again:
}
rcu_read_unlock();
mutex_lock(&res->lock);
- if (res != to_acct(ACCESS_ONCE(ns->bacct))) {
+ if (res != to_acct(READ_ONCE(ns->bacct))) {
mutex_unlock(&res->lock);
acct_put(res);
goto again;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8c3b35f2a269..ca9060cdd6a0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1119,7 +1119,7 @@ perf_event_ctx_lock_nested(struct perf_event *event, int nesting)
again:
rcu_read_lock();
- ctx = ACCESS_ONCE(event->ctx);
+ ctx = READ_ONCE(event->ctx);
if (!atomic_inc_not_zero(&ctx->refcount)) {
rcu_read_unlock();
goto again;
@@ -4881,8 +4881,8 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
if (!rb)
goto aux_unlock;
- aux_offset = ACCESS_ONCE(rb->user_page->aux_offset);
- aux_size = ACCESS_ONCE(rb->user_page->aux_size);
+ aux_offset = READ_ONCE(rb->user_page->aux_offset);
+ aux_size = READ_ONCE(rb->user_page->aux_size);
if (aux_offset < perf_data_size(rb) + PAGE_SIZE)
goto aux_unlock;
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 60be55a64040..b39f4f809b62 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -346,7 +346,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
* (B) <-> (C) ordering is still observed by the pmu driver.
*/
if (!rb->aux_overwrite) {
- aux_tail = ACCESS_ONCE(rb->user_page->aux_tail);
+ aux_tail = READ_ONCE(rb->user_page->aux_tail);
handle->wakeup = local_read(&rb->aux_wakeup) + rb->aux_watermark;
if (aux_head - aux_tail < perf_aux_size(rb))
handle->size = CIRC_SPACE(aux_head, aux_tail, perf_aux_size(rb));
diff --git a/kernel/exit.c b/kernel/exit.c
index fd90195667e1..d0d57d938366 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1294,7 +1294,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
* Ensure that EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE transition
* can't confuse the checks below.
*/
- int exit_state = ACCESS_ONCE(p->exit_state);
+ int exit_state = READ_ONCE(p->exit_state);
int ret;
if (unlikely(exit_state == EXIT_DEAD))
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 9ff173dca1ae..fefa65d102b9 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -63,7 +63,7 @@ static inline struct kthread *to_kthread(struct task_struct *k)
static struct kthread *to_live_kthread(struct task_struct *k)
{
- struct completion *vfork = ACCESS_ONCE(k->vfork_done);
+ struct completion *vfork = READ_ONCE(k->vfork_done);
if (likely(vfork))
return __to_kthread(vfork);
return NULL;
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 53fa971d000d..b16168d6988b 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -29,7 +29,7 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
struct callback_head *head;
do {
- head = ACCESS_ONCE(task->task_works);
+ head = READ_ONCE(task->task_works);
if (unlikely(head == &work_exited))
return -ESRCH;
work->next = head;
@@ -64,7 +64,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
* we raced with task_work_run(), *pprev == NULL/exited.
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
- while ((work = ACCESS_ONCE(*pprev))) {
+ while ((work = READ_ONCE(*pprev))) {
smp_read_barrier_depends();
if (work->func != func)
pprev = &work->next;
@@ -95,7 +95,7 @@ void task_work_run(void)
* work_exited unless the list is empty.
*/
do {
- work = ACCESS_ONCE(task->task_works);
+ work = READ_ONCE(task->task_works);
head = !work && (task->flags & PF_EXITING) ?
&work_exited : NULL;
} while (cmpxchg(&task->task_works, work, head) != work);
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 95181e36891a..c629d0a392a9 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2753,7 +2753,7 @@ rb_reserve_next_event(struct ring_buffer *buffer,
* if it happened, we have to fail the write.
*/
barrier();
- if (unlikely(ACCESS_ONCE(cpu_buffer->buffer) != buffer)) {
+ if (unlikely(READ_ONCE(cpu_buffer->buffer) != buffer)) {
local_dec(&cpu_buffer->committing);
local_dec(&cpu_buffer->commits);
return NULL;
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 3fff4adfd431..7a1a0c512a89 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1154,7 +1154,7 @@ extern struct trace_event_file *find_event_file(struct trace_array *tr,
static inline void *event_file_data(struct file *filp)
{
- return ACCESS_ONCE(file_inode(filp)->i_private);
+ return READ_ONCE(file_inode(filp)->i_private);
}
extern struct mutex event_mutex;
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 2a1abbaca10e..008a28ad652a 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -76,7 +76,7 @@ check_stack(unsigned long ip, unsigned long *stack)
{
unsigned long this_size, flags; unsigned long *p, *top, *start;
static int tracer_frame;
- int frame_size = ACCESS_ONCE(tracer_frame);
+ int frame_size = READ_ONCE(tracer_frame);
int i, x;
this_size = ((unsigned long)stack) & (THREAD_SIZE-1);
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 9bafc211930c..2dc24bcc99f8 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -847,7 +847,7 @@ static bool new_idmap_permitted(const struct file *file,
int proc_setgroups_show(struct seq_file *seq, void *v)
{
struct user_namespace *ns = seq->private;
- unsigned long userns_flags = ACCESS_ONCE(ns->flags);
+ unsigned long userns_flags = READ_ONCE(ns->flags);
seq_printf(seq, "%s\n",
(userns_flags & USERNS_SETGROUPS_ALLOWED) ?
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 9acb29f280ec..0dda0b005927 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -981,7 +981,7 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
goto out;
}
- old = ACCESS_ONCE(watchdog_thresh);
+ old = READ_ONCE(watchdog_thresh);
err = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
if (err || !write)
@@ -990,7 +990,7 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
/*
* Update the sample period. Restore on failure.
*/
- new = ACCESS_ONCE(watchdog_thresh);
+ new = READ_ONCE(watchdog_thresh);
if (old == new)
goto out;
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2232ae3e3ad6..767aa8ae7492 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4550,7 +4550,7 @@ static void rebind_workers(struct worker_pool *pool)
* concurrency management. Note that when or whether
* @worker clears REBOUND doesn't affect correctness.
*
- * ACCESS_ONCE() is necessary because @worker->flags may be
+ * WRITE_ONCE() is necessary because @worker->flags may be
* tested without holding any lock in
* wq_worker_waking_up(). Without it, NOT_RUNNING test may
* fail incorrectly leading to premature concurrency
@@ -4559,7 +4559,7 @@ static void rebind_workers(struct worker_pool *pool)
WARN_ON_ONCE(!(worker_flags & WORKER_UNBOUND));
worker_flags |= WORKER_REBOUND;
worker_flags &= ~WORKER_UNBOUND;
- ACCESS_ONCE(worker->flags) = worker_flags;
+ WRITE_ONCE(worker->flags, worker_flags);
}
spin_unlock_irq(&pool->lock);
--
2.8.1
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