On Thu, May 02, 2019 at 01:42:59PM +0200, Oleg Nesterov wrote:
> On 05/02, Oleg Nesterov wrote:
> >
> > But this all is cosmetic, it seems that we can remove ->rw_sem altogether
> > but I am not sure...
>
> I mean, afaics percpu_down_read() can just do
>
> wait_event(readers_block == 0);
>
> in the slow path, while percpu_down_write()
>
> wait_even_exclusive(xchg(readers_block, 1) == 0);
Cute.. yes that might work.
Although at that point I think we might want to switch readers_block to
atomic_t or something.
> we do not really need ->rw_sem if we rely on wait_queue_head.
>
>
> But in fact, either way it seems that we going to implement another simple
> "non owner" read/write lock, so perhaps we should do this explicitly?
I'd rather not; people might start using it, and then -RT ends up with
another problem.
I mean, percpu-rwsem isn't used in situations where PI really matters,
but rwsem is.
So how about something like so then?
--- a/fs/super.c
+++ b/fs/super.c
@@ -1629,30 +1629,7 @@ EXPORT_SYMBOL(__sb_start_write);
*/
static void sb_wait_write(struct super_block *sb, int level)
{
- percpu_down_write(sb->s_writers.rw_sem + level-1);
-}
-
-/*
- * We are going to return to userspace and forget about these locks, the
- * ownership goes to the caller of thaw_super() which does unlock().
- */
-static void lockdep_sb_freeze_release(struct super_block *sb)
-{
- int level;
-
- for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
- percpu_rwsem_release(sb->s_writers.rw_sem + level, 0,
_THIS_IP_);
-}
-
-/*
- * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
- */
-static void lockdep_sb_freeze_acquire(struct super_block *sb)
-{
- int level;
-
- for (level = 0; level < SB_FREEZE_LEVELS; ++level)
- percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0,
_THIS_IP_);
+ percpu_down_write_non_owner(sb->s_writers.rw_sem + level-1);
}
static void sb_freeze_unlock(struct super_block *sb)
@@ -1660,7 +1637,7 @@ static void sb_freeze_unlock(struct supe
int level;
for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
- percpu_up_write(sb->s_writers.rw_sem + level);
+ percpu_up_write_non_owner(sb->s_writers.rw_sem + level);
}
/**
@@ -1753,7 +1730,6 @@ int freeze_super(struct super_block *sb)
* when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
*/
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
- lockdep_sb_freeze_release(sb);
up_write(&sb->s_umount);
return 0;
}
@@ -1779,14 +1755,11 @@ static int thaw_super_locked(struct supe
goto out;
}
- lockdep_sb_freeze_acquire(sb);
-
if (sb->s_op->unfreeze_fs) {
error = sb->s_op->unfreeze_fs(sb);
if (error) {
printk(KERN_ERR
"VFS:Filesystem thaw failed\n");
- lockdep_sb_freeze_release(sb);
up_write(&sb->s_umount);
return error;
}
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1557,9 +1557,9 @@ void __sb_end_write(struct super_block *
int __sb_start_write(struct super_block *sb, int level, bool wait);
#define __sb_writers_acquired(sb, lev) \
- percpu_rwsem_acquire(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_)
+ percpu_rwsem_acquire(&(sb)->s_writers.rw_sem[(lev)-1], _THIS_IP_)
#define __sb_writers_release(sb, lev) \
- percpu_rwsem_release(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_)
+ percpu_rwsem_release(&(sb)->s_writers.rw_sem[(lev)-1], _THIS_IP_)
/**
* sb_end_write - drop write access to a superblock
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -5,25 +5,34 @@
#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
-#include <linux/rcuwait.h>
+#include <linux/wait.h>
#include <linux/rcu_sync.h>
#include <linux/lockdep.h>
struct percpu_rw_semaphore {
struct rcu_sync rss;
unsigned int __percpu *read_count;
- struct rw_semaphore rw_sem; /* slowpath */
- struct rcuwait writer; /* blocked writer */
- int readers_block;
+ wait_queue_head_t waiters;
+ atomic_t block;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
};
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+#define __PERCPU_RWSEM_DEP_MAP_INIT(lockname) .dep_map = { .name = #lockname
},
+#else
+#define __PERCPU_RWSEM_DEP_MAP_INIT(lockname)
+#endif
+
#define DEFINE_STATIC_PERCPU_RWSEM(name) \
static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_rc_##name); \
static struct percpu_rw_semaphore name = { \
.rss = __RCU_SYNC_INITIALIZER(name.rss, RCU_SCHED_SYNC), \
.read_count = &__percpu_rwsem_rc_##name, \
- .rw_sem = __RWSEM_INITIALIZER(name.rw_sem), \
- .writer = __RCUWAIT_INITIALIZER(name.writer), \
+ .waiters = __WAIT_QUEUE_HEAD_INITIALIZER(name.waiters), \
+ .block = ATOMIC_INIT(0), \
+ __PERCPU_RWSEM_DEP_MAP_INIT(name) \
}
extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
@@ -33,7 +42,7 @@ static inline void percpu_down_read(stru
{
might_sleep();
- rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
+ rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
preempt_disable();
/*
@@ -72,7 +81,7 @@ static inline int percpu_down_read_trylo
*/
if (ret)
- rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);
+ rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
return ret;
}
@@ -89,12 +98,15 @@ static inline void percpu_up_read(struct
__percpu_up_read(sem); /* Unconditional memory barrier */
preempt_enable();
- rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
+ rwsem_release(&sem->dep_map, 1, _RET_IP_);
}
extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);
+extern void percpu_down_write_non_owner(struct percpu_rw_semaphore *);
+extern void percpu_up_write_non_owner(struct percpu_rw_semaphore *);
+
extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
const char *, struct lock_class_key *);
@@ -112,23 +124,15 @@ extern void percpu_free_rwsem(struct per
lockdep_assert_held(&(sem)->rw_sem)
static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
- bool read, unsigned long ip)
+ unsigned long ip)
{
- lock_release(&sem->rw_sem.dep_map, 1, ip);
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
- if (!read)
- sem->rw_sem.owner = RWSEM_OWNER_UNKNOWN;
-#endif
+ lock_release(&sem->dep_map, 1, ip);
}
static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
- bool read, unsigned long ip)
+ unsigned long ip)
{
- lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
- if (!read)
- sem->rw_sem.owner = current;
-#endif
+ lock_acquire(&sem->dep_map, 0, 1, 1, 1, NULL, ip);
}
#endif
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -395,6 +395,9 @@ do {
\
__wait_event_exclusive_cmd(wq_head, condition, cmd1, cmd2);
\
} while (0)
+#define wait_event_exclusive(wq_head, condition)
\
+ wait_event_exclusive_cmd(wq_head, condition, ,)
+
#define __wait_event_cmd(wq_head, condition, cmd1, cmd2)
\
(void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0,
\
cmd1; schedule(); cmd2)
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -1,6 +1,7 @@
#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
+#include <linux/wait.h>
#include <linux/lockdep.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
@@ -16,11 +17,13 @@ int __percpu_init_rwsem(struct percpu_rw
if (unlikely(!sem->read_count))
return -ENOMEM;
- /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
- __init_rwsem(&sem->rw_sem, name, rwsem_key);
- rcuwait_init(&sem->writer);
- sem->readers_block = 0;
+ init_waitqueue_head(&sem->waiters);
+ atomic_set(&sem->block, 0);
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ debug_check_no_locks_freed((void *)sem, sizeof(*sem));
+ lockdep_init_map(&sem->dep_map, name, key, 0);
+#endif
return 0;
}
EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
@@ -63,7 +66,7 @@ int __percpu_down_read(struct percpu_rw_
* If !readers_block the critical section starts here, matched by the
* release in percpu_up_write().
*/
- if (likely(!smp_load_acquire(&sem->readers_block)))
+ if (likely(!atomic_read_acquire(&sem->block)))
return 1;
/*
@@ -80,14 +83,8 @@ int __percpu_down_read(struct percpu_rw_
* and reschedule on the preempt_enable() in percpu_down_read().
*/
preempt_enable_no_resched();
-
- /*
- * Avoid lockdep for the down/up_read() we already have them.
- */
- __down_read(&sem->rw_sem);
+ wait_event(sem->waiters, !atomic_read(&sem->block));
this_cpu_inc(*sem->read_count);
- __up_read(&sem->rw_sem);
-
preempt_disable();
return 1;
}
@@ -104,7 +101,7 @@ void __percpu_up_read(struct percpu_rw_s
__this_cpu_dec(*sem->read_count);
/* Prod writer to recheck readers_active */
- rcuwait_wake_up(&sem->writer);
+ wake_up(&sem->waiters);
}
EXPORT_SYMBOL_GPL(__percpu_up_read);
@@ -139,18 +136,22 @@ static bool readers_active_check(struct
return true;
}
+static inline bool acquire_block(struct percpu_rw_semaphore *sem)
+{
+ if (atomic_read(&sem->block))
+ return false;
+
+ return atomic_xchg(&sem->block, 1) == 0;
+}
+
void percpu_down_write(struct percpu_rw_semaphore *sem)
{
+ rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
+
/* Notify readers to take the slow path. */
rcu_sync_enter(&sem->rss);
- down_write(&sem->rw_sem);
-
- /*
- * Notify new readers to block; up until now, and thus throughout the
- * longish rcu_sync_enter() above, new readers could still come in.
- */
- WRITE_ONCE(sem->readers_block, 1);
+ wait_event_exclusive(sem->waiters, acquire_block(sem));
smp_mb(); /* D matches A */
@@ -161,7 +162,7 @@ void percpu_down_write(struct percpu_rw_
*/
/* Wait for all now active readers to complete. */
- rcuwait_wait_event(&sem->writer, readers_active_check(sem));
+ wait_event(sem->waiters, readers_active_check(sem));
}
EXPORT_SYMBOL_GPL(percpu_down_write);
@@ -177,12 +178,8 @@ void percpu_up_write(struct percpu_rw_se
* Therefore we force it through the slow path which guarantees an
* acquire and thereby guarantees the critical section's consistency.
*/
- smp_store_release(&sem->readers_block, 0);
-
- /*
- * Release the write lock, this will allow readers back in the game.
- */
- up_write(&sem->rw_sem);
+ atomic_set_release(&sem->block, 0);
+ wake_up(&sem->waiters);
/*
* Once this completes (at least one RCU-sched grace period hence) the
@@ -190,5 +187,21 @@ void percpu_up_write(struct percpu_rw_se
* exclusive write lock because its counting.
*/
rcu_sync_exit(&sem->rss);
+
+ rwsem_release(&sem->dep_map, 1, _RET_IP_);
}
EXPORT_SYMBOL_GPL(percpu_up_write);
+
+void percpu_down_write_non_owner(struct percpu_rw_semaphore *sem)
+{
+ percpu_down_write(sem);
+ rwsem_release(&sem->dep_map, 1, _RET_IP_);
+}
+EXPORT_SYMBOL_GPL(percpu_down_write_non_owner);
+
+void percpu_up_write_non_owner(struct percpu_rw_semaphore *sem)
+{
+ rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
+ percpu_up_write(sem);
+}
+EXPORT_SYMBOL_GPL(percpu_up_write_non_owner);
