Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem

Peter Zijlstra
2019-10-30T17:52:39Z

On Tue, Oct 29, 2019 at 07:47:39PM +0100, Peter Zijlstra wrote:
> I've made these changes. Now let me go have a play with that second
> waitqueue.

What I've ended up with is a 'custom' waitqueue and an rcuwait. The
rcuwait conveniently got around the tedious preempt_enable/disable
around the __percpu_up_read() wakeup.

I realized that up_read will only ever have to wake a (single) blocked
writer, never a series of readers.

Compile tested only, I'll build and boot test once i've had dinner.

---
include/linux/percpu-rwsem.h | 64 +++++++++--------
include/linux/wait.h | 1 +
kernel/cpu.c | 4 +-
kernel/locking/percpu-rwsem.c | 156 ++++++++++++++++++++++++++++++------------
4 files changed, 150 insertions(+), 75 deletions(-)

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index ad2ca2a89d5b..806af4bf257e 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -6,38 +6,51 @@
#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;
+ struct rcuwait writer;
+ 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_PERCPU_RWSEM(name, is_static) \
static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_rc_##name); \
is_static struct percpu_rw_semaphore name = { \
.rss = __RCU_SYNC_INITIALIZER(name.rss), \
.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) \
}
+
#define DEFINE_PERCPU_RWSEM(name) \
__DEFINE_PERCPU_RWSEM(name, /* not static */)
#define DEFINE_STATIC_PERCPU_RWSEM(name) \
__DEFINE_PERCPU_RWSEM(name, static)

-extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
+extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);
extern void __percpu_up_read(struct percpu_rw_semaphore *);

static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
{
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();
/*
@@ -48,8 +61,9 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
* and that once the synchronize_rcu() is done, the writer will see
* anything we did within this RCU-sched read-size critical section.
*/
- __this_cpu_inc(*sem->read_count);
- if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+ if (likely(rcu_sync_is_idle(&sem->rss)))
+ __this_cpu_inc(*sem->read_count);
+ else
__percpu_down_read(sem, false); /* Unconditional memory barrier */
/*
* The preempt_enable() prevents the compiler from
@@ -58,16 +72,17 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
preempt_enable();
}

-static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
+static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
- int ret = 1;
+ bool ret = true;

preempt_disable();
/*
* Same as in percpu_down_read().
*/
- __this_cpu_inc(*sem->read_count);
- if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+ if (likely(!rcu_sync_is_idle(&sem->rss)))
+ __this_cpu_inc(*sem->read_count);
+ else
ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
preempt_enable();
/*
@@ -76,13 +91,15 @@ static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
*/

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;
}

static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
{
+ rwsem_release(&sem->dep_map, _RET_IP_);
+
preempt_disable();
/*
* Same as in percpu_down_read().
@@ -91,9 +108,8 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
__this_cpu_dec(*sem->read_count);
else
__percpu_up_read(sem); /* Unconditional memory barrier */
- preempt_enable();

- rwsem_release(&sem->rw_sem.dep_map, _RET_IP_);
+ preempt_enable();
}

extern void percpu_down_write(struct percpu_rw_semaphore *);
@@ -110,29 +126,19 @@ extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
__percpu_init_rwsem(sem, #sem, &rwsem_key); \
})

-#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)
-
-#define percpu_rwsem_assert_held(sem) \
- lockdep_assert_held(&(sem)->rw_sem)
+#define percpu_rwsem_is_held(sem) lockdep_is_held(sem)
+#define percpu_rwsem_assert_held(sem) lockdep_assert_held(sem)

static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
bool read, unsigned long ip)
{
- lock_release(&sem->rw_sem.dep_map, ip);
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
- if (!read)
- atomic_long_set(&sem->rw_sem.owner, RWSEM_OWNER_UNKNOWN);
-#endif
+ lock_release(&sem->dep_map, ip);
}

static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
bool read, unsigned long ip)
{
- lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
-#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
- if (!read)
- atomic_long_set(&sem->rw_sem.owner, (long)current);
-#endif
+ lock_acquire(&sem->dep_map, 0, 1, read, 1, NULL, ip);
}

#endif
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 3eb7cae8206c..94580163a4b1 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -20,6 +20,7 @@ int default_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int
#define WQ_FLAG_EXCLUSIVE 0x01
#define WQ_FLAG_WOKEN 0x02
#define WQ_FLAG_BOOKMARK 0x04
+#define WQ_FLAG_CUSTOM 0x08

/*
* A single wait-queue entry structure:
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 68f85627d909..7ea0c0225590 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -331,12 +331,12 @@ void lockdep_assert_cpus_held(void)

static void lockdep_acquire_cpus_lock(void)
{
- rwsem_acquire(&cpu_hotplug_lock.rw_sem.dep_map, 0, 0, _THIS_IP_);
+ rwsem_acquire(&cpu_hotplug_lock.dep_map, 0, 0, _THIS_IP_);
}

static void lockdep_release_cpus_lock(void)
{
- rwsem_release(&cpu_hotplug_lock.rw_sem.dep_map, _THIS_IP_);
+ rwsem_release(&cpu_hotplug_lock.dep_map, _THIS_IP_);
}

/*
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 364d38a0c444..9aa69345cdc8 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -2,6 +2,7 @@
#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>
@@ -11,17 +12,20 @@
#include "rwsem.h"

int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
- const char *name, struct lock_class_key *rwsem_key)
+ const char *name, struct lock_class_key *key)
{
sem->read_count = alloc_percpu(int);
if (unlikely(!sem->read_count))
return -ENOMEM;

- /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
rcu_sync_init(&sem->rss);
- __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);
@@ -41,31 +45,95 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
}
EXPORT_SYMBOL_GPL(percpu_free_rwsem);

-int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
+enum wake_state {
+ unknown = -1,
+ writer = 0,
+ reader = 1,
+};
+
+/*
+ * percpu_rwsem_wake_function -- provide FIFO fair reader/writer wakeups
+ *
+ * As per percpu_rwsem_wait() all waiters are queued exclusive (tail/FIFO)
+ * without autoremove to preserve FIFO order.
+ */
+static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
+ unsigned int mode, int wake_flags,
+ void *key)
+{
+ enum wake_state state = (wq_entry->flags & WQ_FLAG_CUSTOM) ? reader : writer;
+ enum wake_state *statep = key;
+
+ if (*statep != unknown && (*statep == writer || state == writer))
+ return 1; /* stop; woken 1 writer or exhausted readers */
+
+ if (default_wake_function(wq_entry, mode, wake_flags, NULL))
+ *statep = state;
+
+ return 0; /* continue waking */
+}
+
+#define percpu_rwsem_wait(sem, reader, cond) \
+do { \
+ DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function); \
+ \
+ if (reader) \
+ wq_entry.flags |= WQ_FLAG_CUSTOM; \
+ \
+ add_wait_queue_exclusive(&(sem)->waiters, &wq_entry); \
+ for (;;) { \
+ set_current_state(TASK_UNINTERRUPTIBLE); \
+ if (cond) \
+ break; \
+ schedule(); \
+ } \
+ __set_current_state(TASK_RUNNING); \
+ remove_wait_queue(&(sem)->waiters, &wq_entry); \
+} while (0)
+
+#define percpu_rwsem_wake(sem) \
+do { \
+ enum wake_state ____state = unknown; \
+ __wake_up(&(sem)->waiters, TASK_NORMAL, 1, &____state); \
+} while (0)
+
+bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
{
+ if (atomic_read(&sem->block)) {
+again:
+ if (try)
+ return false;
+
+ preempt_enable();
+ percpu_rwsem_wait(sem, reader, !atomic_read(&sem->block));
+ preempt_disable();
+ }
+
/*
* Due to having preemption disabled the decrement happens on
* the same CPU as the increment, avoiding the
* increment-on-one-CPU-and-decrement-on-another problem.
*
- * If the reader misses the writer's assignment of readers_block, then
- * the writer is guaranteed to see the reader's increment.
+ * If the reader misses the writer's assignment of sem->block, then the
+ * writer is guaranteed to see the reader's increment.
*
* Conversely, any readers that increment their sem->read_count after
- * the writer looks are guaranteed to see the readers_block value,
- * which in turn means that they are guaranteed to immediately
- * decrement their sem->read_count, so that it doesn't matter that the
- * writer missed them.
+ * the writer looks are guaranteed to see the sem->block value, which
+ * in turn means that they are guaranteed to immediately decrement
+ * their sem->read_count, so that it doesn't matter that the writer
+ * missed them.
*/

+ __this_cpu_inc(*sem->read_count);
+
smp_mb(); /* A matches D */

/*
- * If !readers_block the critical section starts here, matched by the
+ * If !sem->block the critical section starts here, matched by the
* release in percpu_up_write().
*/
- if (likely(!smp_load_acquire(&sem->readers_block)))
- return 1;
+ if (likely(!atomic_read_acquire(&sem->block)))
+ return true;

/*
* Per the above comment; we still have preemption disabled and
@@ -73,24 +141,12 @@ int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
*/
__percpu_up_read(sem);

- if (try)
- return 0;
-
- /*
- * We either call schedule() in the wait, or we'll fall through
- * 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.
+ * percpu_down_write() could've set sem->block right after we've seen
+ * it 0 but missed our this_cpu_inc(), which is exactly the condition
+ * we get called for from percpu_down_read().
*/
- __down_read(&sem->rw_sem);
- this_cpu_inc(*sem->read_count);
- __up_read(&sem->rw_sem);
-
- preempt_disable();
- return 1;
+ goto again;
}
EXPORT_SYMBOL_GPL(__percpu_down_read);

@@ -104,7 +160,7 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
*/
__this_cpu_dec(*sem->read_count);

- /* Prod writer to recheck readers_active */
+ /* Prod writer to re-evaluate readers_active_check() */
rcuwait_wake_up(&sem->writer);
}
EXPORT_SYMBOL_GPL(__percpu_up_read);
@@ -124,6 +180,8 @@ EXPORT_SYMBOL_GPL(__percpu_up_read);
* zero. If this sum is zero, then it is stable due to the fact that if any
* newly arriving readers increment a given counter, they will immediately
* decrement that same counter.
+ *
+ * Assumes sem->block is set.
*/
static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
@@ -140,28 +198,36 @@ static bool readers_active_check(struct percpu_rw_semaphore *sem)
return true;
}

+static inline bool try_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.
+ * Try set sem->block; this provides writer-writer exclusion.
+ * Having sem->block set makes new readers block.
*/
- WRITE_ONCE(sem->readers_block, 1);
+ percpu_rwsem_wait(sem, writer, try_acquire_block(sem));

- smp_mb(); /* D matches A */
+ /* smp_mb() implied by try_acquire_block() on success -- D matches A */

/*
- * If they don't see our writer of readers_block, then we are
- * guaranteed to see their sem->read_count increment, and therefore
- * will wait for them.
+ * If they don't see our store of sem->block, then we are guaranteed to
+ * see their sem->read_count increment, and therefore will wait for
+ * them.
*/

- /* Wait for all now active readers to complete. */
+ /* Wait for all active readers to complete. */
rcuwait_wait_event(&sem->writer, readers_active_check(sem));
}
EXPORT_SYMBOL_GPL(percpu_down_write);
@@ -178,12 +244,12 @@ void percpu_up_write(struct percpu_rw_semaphore *sem)
* 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);
+ atomic_set_release(&sem->block, 0);

/*
- * Release the write lock, this will allow readers back in the game.
+ * Prod any pending reader/writer to make progress.
*/
- up_write(&sem->rw_sem);
+ percpu_rwsem_wake(sem);

/*
* Once this completes (at least one RCU-sched grace period hence) the
@@ -191,5 +257,7 @@ void percpu_up_write(struct percpu_rw_semaphore *sem)
* exclusive write lock because its counting.
*/
rcu_sync_exit(&sem->rss);
+
+ rwsem_release(&sem->dep_map, _RET_IP_);
}
EXPORT_SYMBOL_GPL(percpu_up_write);