Re: [PATCH 2/3] locking/barriers: Kill lockless_dereference

[Paul E. McKenney]

On Thu, Oct 12, 2017 at 03:26:16PM +0100, Will Deacon wrote:
> lockless_dereference is a nice idea, but its 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>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

> ---
>  Documentation/memory-barriers.txt                   | 12 ------------
>  .../translations/ko_KR/memory-barriers.txt          | 12 ------------
>  arch/x86/events/core.c                              |  2 +-
>  arch/x86/include/asm/mmu_context.h                  |  4 ++--
>  arch/x86/kernel/ldt.c                               |  2 +-
>  drivers/md/dm-mpath.c                               | 20 ++++++++++----------
>  fs/dcache.c                                         |  4 ++--
>  fs/overlayfs/ovl_entry.h                            |  2 +-
>  fs/overlayfs/readdir.c                              |  2 +-
>  include/linux/compiler.h                            | 21 +--------------------
>  include/linux/rculist.h                             |  4 ++--
>  include/linux/rcupdate.h                            |  4 ++--
>  kernel/events/core.c                                |  4 ++--
>  kernel/seccomp.c                                    |  2 +-
>  kernel/task_work.c                                  |  2 +-
>  mm/slab.h                                           |  2 +-
>  16 files changed, 28 insertions(+), 71 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index b759a60624fd..470a682f3fa4 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1886,18 +1886,6 @@ 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 25d9b5adcd42..36b49bd09264 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -77,5 +77,5 @@ 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 0f85ee9c3268..c67a7703296b 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 c714239f453f..4517662f128c 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -242,6 +242,7 @@ 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)
> @@ -345,24 +346,4 @@ 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 b1fd8bf85fdc..3a2bb7d8ed4d 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 de50d8a4cf41..380a3aeb09d7 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;
> -- 
> 2.1.4
>