Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables

From: Paul E. McKenney
Date: Thu Oct 05 2017 - 15:22:38 EST

On Thu, Oct 05, 2017 at 05:31:54PM +0100, Will Deacon wrote:
> Hi Paul,
>
> On Tue, Oct 03, 2017 at 12:11:10PM -0700, Paul E. McKenney wrote:
> > On Fri, Sep 29, 2017 at 05:33:49PM +0100, Will Deacon wrote:
> > > On Fri, Sep 29, 2017 at 09:29:39AM -0700, Paul E. McKenney wrote:
> > > > On Fri, Sep 29, 2017 at 10:08:43AM +0100, Will Deacon wrote:
> > > > > Ok, but where does that leave us wrt my initial proposal of moving
> > > > > smp_read_barrier_depends() into READ_ONCE and getting rid of
> > > > > lockless_dereference?
> > > > >
> > > > > Michael (or anybody else running mainline on SMP Alpha) -- would you be
> > > > > able to give the diff below a spin and see whether there's a measurable
> > > > > performance impact?
> > > >
> > > > This will be a sensitive test. The smp_read_barrier_depends() can be
> > > > removed from lockless_dereference(). Without this removal Alpha will
> > > > get two memory barriers from rcu_dereference() and friends.
> > >
> > > Oh yes, good point. I was trying to keep the diff simple, but you're
> > > right that this is packing too many barriers. Fixed diff below.
> >
> > Not seeing any objections thus far. If there are none by (say) the
> > end of this week, I would be happy to queue a patch for the 4.16
> > merge window. That should give ample opportunity for further review
> > and testing.
>
> Ok, full patch below.

Very good, applied for testing and review. I did have to adjust context
a bit for other -rcu commits, and the result is below. (Removed a single
"*" in a comment.)

Thanx, Paul

------------------------------------------------------------------------

commit 7e3675cc18bbf4d84f60bfc02ff563ae3764ad35
Author: Will Deacon <will.deacon@xxxxxxx>
Date: Thu Oct 5 17:31:54 2017 +0100

locking/barriers: Kill lockless_dereference

lockless_dereference is a nice idea, but it's gained little traction in
kernel code since it's introduction three years ago. This is partly
because it's a pain to type, but also because using READ_ONCE instead
will work correctly on all architectures apart from Alpha, which is a
fully supported but somewhat niche architecture these days.

This patch moves smp_read_barrier_depends() (a NOP on all architectures
other than Alpha) from lockless_dereference into READ_ONCE, converts
the few actual users over to READ_ONCE and then finally removes
lockless_dereference altogether.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 519940ec767f..479ecec80593 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1880,18 +1880,6 @@ There are some more advanced barrier functions:
See Documentation/atomic_{t,bitops}.txt for more information.


- (*) lockless_dereference();
-
- This can be thought of as a pointer-fetch wrapper around the
- smp_read_barrier_depends() data-dependency barrier.
-
- This is also similar to rcu_dereference(), but in cases where
- object lifetime is handled by some mechanism other than RCU, for
- example, when the objects removed only when the system goes down.
- In addition, lockless_dereference() is used in some data structures
- that can be used both with and without RCU.
-
-
(*) dma_wmb();
(*) dma_rmb();

diff --git a/Documentation/translations/ko_KR/memory-barriers.txt b/Documentation/translations/ko_KR/memory-barriers.txt
index a7a813258013..ec3b46e27b7a 100644
--- a/Documentation/translations/ko_KR/memory-barriers.txt
+++ b/Documentation/translations/ko_KR/memory-barriers.txt
@@ -1858,18 +1858,6 @@ Mandatory ëëìëì SMP ììíììë UP ììíììë SMP í
ìêíìì.


- (*) lockless_dereference();
-
- ì íìë smp_read_barrier_depends() ëìí ììì ëëìë ììíë
- íìí ìììê ëí(wrapper) íìë ìêë ì ììëë.
-
- êìì ëìííìì RCU ìì ëìëììë êëëëë ìì ììíë
- rcu_dereference() ìë ììíë, ìë ëë êìê ììíì êì ëìë
- ìêëë êì ëìëë. ëí, lockless_dereference() ì RCU ì íê
- ììëìë, RCU ìì ììë ìë ìë ìë ëìí êìì ììëê
- ììëë.
-
-
(*) dma_wmb();
(*) dma_rmb();

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 80534d3c2480..589af1eec7c1 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2371,7 +2371,7 @@ static unsigned long get_segment_base(unsigned int segment)
struct ldt_struct *ldt;

/* IRQs are off, so this synchronizes with smp_store_release */
- ldt = lockless_dereference(current->active_mm->context.ldt);
+ ldt = READ_ONCE(current->active_mm->context.ldt);
if (!ldt || idx >= ldt->nr_entries)
return 0;

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index c120b5db178a..9037a4e546e8 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -72,8 +72,8 @@ static inline void load_mm_ldt(struct mm_struct *mm)
#ifdef CONFIG_MODIFY_LDT_SYSCALL
struct ldt_struct *ldt;

- /* lockless_dereference synchronizes with smp_store_release */
- ldt = lockless_dereference(mm->context.ldt);
+ /* READ_ONCE synchronizes with smp_store_release */
+ ldt = READ_ONCE(mm->context.ldt);

/*
* Any change to mm->context.ldt is followed by an IPI to all
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index f0e64db18ac8..0a21390642c4 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -101,7 +101,7 @@ static void finalize_ldt_struct(struct ldt_struct *ldt)
static void install_ldt(struct mm_struct *current_mm,
struct ldt_struct *ldt)
{
- /* Synchronizes with lockless_dereference in load_mm_ldt. */
+ /* Synchronizes with READ_ONCE in load_mm_ldt. */
smp_store_release(&current_mm->context.ldt, ldt);

/* Activate the LDT for all CPUs using current_mm. */
diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 11f273d2f018..3f88c9d32f7e 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -366,7 +366,7 @@ static struct pgpath *choose_path_in_pg(struct multipath *m,

pgpath = path_to_pgpath(path);

- if (unlikely(lockless_dereference(m->current_pg) != pg)) {
+ if (unlikely(READ_ONCE(m->current_pg) != pg)) {
/* Only update current_pgpath if pg changed */
spin_lock_irqsave(&m->lock, flags);
m->current_pgpath = pgpath;
@@ -390,7 +390,7 @@ static struct pgpath *choose_pgpath(struct multipath *m, size_t nr_bytes)
}

/* Were we instructed to switch PG? */
- if (lockless_dereference(m->next_pg)) {
+ if (READ_ONCE(m->next_pg)) {
spin_lock_irqsave(&m->lock, flags);
pg = m->next_pg;
if (!pg) {
@@ -406,7 +406,7 @@ static struct pgpath *choose_pgpath(struct multipath *m, size_t nr_bytes)

/* Don't change PG until it has no remaining paths */
check_current_pg:
- pg = lockless_dereference(m->current_pg);
+ pg = READ_ONCE(m->current_pg);
if (pg) {
pgpath = choose_path_in_pg(m, pg, nr_bytes);
if (!IS_ERR_OR_NULL(pgpath))
@@ -473,7 +473,7 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
struct request *clone;

/* Do we need to select a new pgpath? */
- pgpath = lockless_dereference(m->current_pgpath);
+ pgpath = READ_ONCE(m->current_pgpath);
if (!pgpath || !test_bit(MPATHF_QUEUE_IO, &m->flags))
pgpath = choose_pgpath(m, nr_bytes);

@@ -535,7 +535,7 @@ static int __multipath_map_bio(struct multipath *m, struct bio *bio, struct dm_m
bool queue_io;

/* Do we need to select a new pgpath? */
- pgpath = lockless_dereference(m->current_pgpath);
+ pgpath = READ_ONCE(m->current_pgpath);
queue_io = test_bit(MPATHF_QUEUE_IO, &m->flags);
if (!pgpath || !queue_io)
pgpath = choose_pgpath(m, nr_bytes);
@@ -1804,7 +1804,7 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
struct pgpath *current_pgpath;
int r;

- current_pgpath = lockless_dereference(m->current_pgpath);
+ current_pgpath = READ_ONCE(m->current_pgpath);
if (!current_pgpath)
current_pgpath = choose_pgpath(m, 0);

@@ -1826,7 +1826,7 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
}

if (r == -ENOTCONN) {
- if (!lockless_dereference(m->current_pg)) {
+ if (!READ_ONCE(m->current_pg)) {
/* Path status changed, redo selection */
(void) choose_pgpath(m, 0);
}
@@ -1895,9 +1895,9 @@ static int multipath_busy(struct dm_target *ti)
return (m->queue_mode != DM_TYPE_MQ_REQUEST_BASED);

/* Guess which priority_group will be used at next mapping time */
- pg = lockless_dereference(m->current_pg);
- next_pg = lockless_dereference(m->next_pg);
- if (unlikely(!lockless_dereference(m->current_pgpath) && next_pg))
+ pg = READ_ONCE(m->current_pg);
+ next_pg = READ_ONCE(m->next_pg);
+ if (unlikely(!READ_ONCE(m->current_pgpath) && next_pg))
pg = next_pg;

if (!pg) {
diff --git a/fs/dcache.c b/fs/dcache.c
index f90141387f01..34c852af215c 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -231,7 +231,7 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
{
/*
* Be careful about RCU walk racing with rename:
- * use 'lockless_dereference' to fetch the name pointer.
+ * use 'READ_ONCE' to fetch the name pointer.
*
* NOTE! Even if a rename will mean that the length
* was not loaded atomically, we don't care. The
@@ -245,7 +245,7 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
* early because the data cannot match (there can
* be no NUL in the ct/tcount data)
*/
- const unsigned char *cs = lockless_dereference(dentry->d_name.name);
+ const unsigned char *cs = READ_ONCE(dentry->d_name.name);

return dentry_string_cmp(cs, ct, tcount);
}
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 878a750986dd..0f6809fa6628 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -74,5 +74,5 @@ static inline struct ovl_inode *OVL_I(struct inode *inode)

static inline struct dentry *ovl_upperdentry_dereference(struct ovl_inode *oi)
{
- return lockless_dereference(oi->__upperdentry);
+ return READ_ONCE(oi->__upperdentry);
}
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 62e9b22a2077..0b389d330613 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -754,7 +754,7 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
if (!od->is_upper && OVL_TYPE_UPPER(ovl_path_type(dentry))) {
struct inode *inode = file_inode(file);

- realfile = lockless_dereference(od->upperfile);
+ realfile = READ_ONCE(od->upperfile);
if (!realfile) {
struct path upperpath;

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e95a2631e545..f260ff39f90f 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -340,6 +340,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
__read_once_size(&(x), __u.__c, sizeof(x)); \
else \
__read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \
+ smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
__u.__val; \
})
#define READ_ONCE(x) __READ_ONCE(x, 1)
@@ -604,24 +605,4 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
(volatile typeof(x) *)&(x); })
#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))

-/**
- * lockless_dereference() - safely load a pointer for later dereference
- * @p: The pointer to load
- *
- * Similar to rcu_dereference(), but for situations where the pointed-to
- * object's lifetime is managed by something other than RCU. That
- * "something other" might be reference counting or simple immortality.
- *
- * The seemingly unused variable ___typecheck_p validates that @p is
- * indeed a pointer type by using a pointer to typeof(*p) as the type.
- * Taking a pointer to typeof(*p) again is needed in case p is void *.
- */
-#define lockless_dereference(p) \
-({ \
- typeof(p) _________p1 = READ_ONCE(p); \
- typeof(*(p)) *___typecheck_p __maybe_unused; \
- smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
- (_________p1); \
-})
-
#endif /* __LINUX_COMPILER_H */
diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 2bea1d5e9930..5ed091c064b2 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -274,7 +274,7 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
* primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
*/
#define list_entry_rcu(ptr, type, member) \
- container_of(lockless_dereference(ptr), type, member)
+ container_of(READ_ONCE(ptr), type, member)

/*
* Where are list_empty_rcu() and list_first_entry_rcu()?
@@ -367,7 +367,7 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
* example is when items are added to the list, but never deleted.
*/
#define list_entry_lockless(ptr, type, member) \
- container_of((typeof(ptr))lockless_dereference(ptr), type, member)
+ container_of((typeof(ptr))READ_ONCE(ptr), type, member)

/**
* list_for_each_entry_lockless - iterate over rcu list of given type
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 1a9f70d44af9..a6ddc42f87a5 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -346,7 +346,7 @@ static inline void rcu_preempt_sleep_check(void) { }
#define __rcu_dereference_check(p, c, space) \
({ \
/* Dependency order vs. p above. */ \
- typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
+ typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
rcu_dereference_sparse(p, space); \
((typeof(*p) __force __kernel *)(________p1)); \
@@ -360,7 +360,7 @@ static inline void rcu_preempt_sleep_check(void) { }
#define rcu_dereference_raw(p) \
({ \
/* Dependency order vs. p above. */ \
- typeof(p) ________p1 = lockless_dereference(p); \
+ typeof(p) ________p1 = READ_ONCE(p); \
((typeof(*p) __force __kernel *)(________p1)); \
})

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6bc21e202ae4..417812ce0099 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4231,7 +4231,7 @@ static void perf_remove_from_owner(struct perf_event *event)
* indeed free this event, otherwise we need to serialize on
* owner->perf_event_mutex.
*/
- owner = lockless_dereference(event->owner);
+ owner = READ_ONCE(event->owner);
if (owner) {
/*
* Since delayed_put_task_struct() also drops the last
@@ -4328,7 +4328,7 @@ int perf_event_release_kernel(struct perf_event *event)
* Cannot change, child events are not migrated, see the
* comment with perf_event_ctx_lock_nested().
*/
- ctx = lockless_dereference(child->ctx);
+ ctx = READ_ONCE(child->ctx);
/*
* Since child_mutex nests inside ctx::mutex, we must jump
* through hoops. We start by grabbing a reference on the ctx.
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index bb3a38005b9c..1daa8b61a268 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -189,7 +189,7 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
u32 ret = SECCOMP_RET_ALLOW;
/* Make sure cross-thread synced filter points somewhere sane. */
struct seccomp_filter *f =
- lockless_dereference(current->seccomp.filter);
+ READ_ONCE(current->seccomp.filter);

/* Ensure unexpected behavior doesn't result in failing open. */
if (unlikely(WARN_ON(f == NULL)))
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 836a72a66fba..9a9f262fc53d 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -67,7 +67,7 @@ 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 = lockless_dereference(*pprev))) {
+ while ((work = READ_ONCE(*pprev))) {
if (work->func != func)
pprev = &work->next;
else if (cmpxchg(pprev, work, work->next) == work)
diff --git a/mm/slab.h b/mm/slab.h
index 073362816acc..8894f811a89d 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -258,7 +258,7 @@ cache_from_memcg_idx(struct kmem_cache *s, int idx)
* memcg_caches issues a write barrier to match this (see
* memcg_create_kmem_cache()).
*/
- cachep = lockless_dereference(arr->entries[idx]);
+ cachep = READ_ONCE(arr->entries[idx]);
rcu_read_unlock();

return cachep;

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