Re: [PATCH v3 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily

[Oleg Nesterov <oleg@redhat.com>]

On 11/07, Mikulas Patocka wrote:
>
> It looks sensible.
>
> Here I'm sending an improvement of the patch - I changed it so that there
> are not two-level nested functions for the fast path and so that both
> percpu_down_read and percpu_up_read use the same piece of code (to reduce
> cache footprint).

IOW, the only change is that you eliminate "static update_fast_ctr()"
and fold it into down/up_read which takes the additional argument.

Honestly, personally I do not think this is better, but I won't argue.
I agree with everything but I guess we need the ack from Paul.

As for EXPORT_SYMBOL, I do not mind of course. But currently the only
user is block_dev.c.

> Currently the writer does msleep() plus synchronize_sched() 3 times
> to acquire/release the semaphore, and during this time the readers
> are blocked completely. Even if the "write" section was not actually
> started or if it was already finished.
> 
> With this patch down_write/up_write does synchronize_sched() twice
> and down_read/up_read are still possible during this time, just they
> use the slow path.
> 
> percpu_down_write() first forces the readers to use rw_semaphore and
> increment the "slow" counter to take the lock for reading, then it
> takes that rw_semaphore for writing and blocks the readers.
> 
> Also. With this patch the code relies on the documented behaviour of
> synchronize_sched(), it doesn't try to pair synchronize_sched() with
> barrier.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> ---
>  include/linux/percpu-rwsem.h |   80 ++++++-------------------------
>  lib/Makefile                 |    2 
>  lib/percpu-rwsem.c           |  110 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 127 insertions(+), 65 deletions(-)
>  create mode 100644 lib/percpu-rwsem.c
> 
> Index: linux-3.6.6-fast/include/linux/percpu-rwsem.h
> ===================================================================
> --- linux-3.6.6-fast.orig/include/linux/percpu-rwsem.h	2012-11-05 16:21:29.000000000 +0100
> +++ linux-3.6.6-fast/include/linux/percpu-rwsem.h	2012-11-07 16:44:04.000000000 +0100
> @@ -2,82 +2,34 @@
>  #define _LINUX_PERCPU_RWSEM_H
>  
>  #include <linux/mutex.h>
> +#include <linux/rwsem.h>
>  #include <linux/percpu.h>
> -#include <linux/rcupdate.h>
> -#include <linux/delay.h>
> +#include <linux/wait.h>
>  
>  struct percpu_rw_semaphore {
> -	unsigned __percpu *counters;
> -	bool locked;
> -	struct mutex mtx;
> +	unsigned int __percpu	*fast_read_ctr;
> +	struct mutex		writer_mutex;
> +	struct rw_semaphore	rw_sem;
> +	atomic_t		slow_read_ctr;
> +	wait_queue_head_t	write_waitq;
>  };
>  
> -#define light_mb()	barrier()
> -#define heavy_mb()	synchronize_sched()
> +extern void __percpu_down_up_read(struct percpu_rw_semaphore *, int);
>  
> -static inline void percpu_down_read(struct percpu_rw_semaphore *p)
> -{
> -	rcu_read_lock_sched();
> -	if (unlikely(p->locked)) {
> -		rcu_read_unlock_sched();
> -		mutex_lock(&p->mtx);
> -		this_cpu_inc(*p->counters);
> -		mutex_unlock(&p->mtx);
> -		return;
> -	}
> -	this_cpu_inc(*p->counters);
> -	rcu_read_unlock_sched();
> -	light_mb(); /* A, between read of p->locked and read of data, paired with D */
> -}
> -
> -static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> -{
> -	light_mb(); /* B, between read of the data and write to p->counter, paired with C */
> -	this_cpu_dec(*p->counters);
> -}
> -
> -static inline unsigned __percpu_count(unsigned __percpu *counters)
> -{
> -	unsigned total = 0;
> -	int cpu;
> -
> -	for_each_possible_cpu(cpu)
> -		total += ACCESS_ONCE(*per_cpu_ptr(counters, cpu));
> +extern void percpu_down_write(struct percpu_rw_semaphore *);
> +extern void percpu_up_write(struct percpu_rw_semaphore *);
>  
> -	return total;
> -}
> -
> -static inline void percpu_down_write(struct percpu_rw_semaphore *p)
> -{
> -	mutex_lock(&p->mtx);
> -	p->locked = true;
> -	synchronize_sched(); /* make sure that all readers exit the rcu_read_lock_sched region */
> -	while (__percpu_count(p->counters))
> -		msleep(1);
> -	heavy_mb(); /* C, between read of p->counter and write to data, paired with B */
> -}
> -
> -static inline void percpu_up_write(struct percpu_rw_semaphore *p)
> -{
> -	heavy_mb(); /* D, between write to data and write to p->locked, paired with A */
> -	p->locked = false;
> -	mutex_unlock(&p->mtx);
> -}
> +extern int percpu_init_rwsem(struct percpu_rw_semaphore *);
> +extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
>  
> -static inline int percpu_init_rwsem(struct percpu_rw_semaphore *p)
> +static inline void percpu_down_read(struct percpu_rw_semaphore *s)
>  {
> -	p->counters = alloc_percpu(unsigned);
> -	if (unlikely(!p->counters))
> -		return -ENOMEM;
> -	p->locked = false;
> -	mutex_init(&p->mtx);
> -	return 0;
> +	__percpu_down_up_read(s, 1);
>  }
>  
> -static inline void percpu_free_rwsem(struct percpu_rw_semaphore *p)
> +static inline void percpu_up_read(struct percpu_rw_semaphore *s)
>  {
> -	free_percpu(p->counters);
> -	p->counters = NULL; /* catch use after free bugs */
> +	__percpu_down_up_read(s, -1);
>  }
>  
>  #endif
> Index: linux-3.6.6-fast/lib/Makefile
> ===================================================================
> --- linux-3.6.6-fast.orig/lib/Makefile	2012-10-02 00:47:57.000000000 +0200
> +++ linux-3.6.6-fast/lib/Makefile	2012-11-07 03:10:44.000000000 +0100
> @@ -12,7 +12,7 @@ lib-y := ctype.o string.o vsprintf.o cmd
>  	 idr.o int_sqrt.o extable.o prio_tree.o \
>  	 sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \
>  	 proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \
> -	 is_single_threaded.o plist.o decompress.o
> +	 is_single_threaded.o plist.o decompress.o percpu-rwsem.o
>  
>  lib-$(CONFIG_MMU) += ioremap.o
>  lib-$(CONFIG_SMP) += cpumask.o
> Index: linux-3.6.6-fast/lib/percpu-rwsem.c
> ===================================================================
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ linux-3.6.6-fast/lib/percpu-rwsem.c	2012-11-07 16:43:27.000000000 +0100
> @@ -0,0 +1,110 @@
> +#include <linux/percpu-rwsem.h>
> +#include <linux/rcupdate.h>
> +#include <linux/sched.h>
> +#include <linux/module.h>
> +
> +int percpu_init_rwsem(struct percpu_rw_semaphore *brw)
> +{
> +	brw->fast_read_ctr = alloc_percpu(int);
> +	if (unlikely(!brw->fast_read_ctr))
> +		return -ENOMEM;
> +
> +	mutex_init(&brw->writer_mutex);
> +	init_rwsem(&brw->rw_sem);
> +	atomic_set(&brw->slow_read_ctr, 0);
> +	init_waitqueue_head(&brw->write_waitq);
> +	return 0;
> +}
> +EXPORT_SYMBOL(percpu_init_rwsem);
> +
> +void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
> +{
> +	free_percpu(brw->fast_read_ctr);
> +	brw->fast_read_ctr = NULL; /* catch use after free bugs */
> +}
> +EXPORT_SYMBOL(percpu_free_rwsem);
> +
> +void __percpu_down_up_read(struct percpu_rw_semaphore *brw, int val)
> +{
> +	preempt_disable();
> +	if (likely(!mutex_is_locked(&brw->writer_mutex))) {
> +		__this_cpu_add(*brw->fast_read_ctr, val);
> +		preempt_enable();
> +		return;
> +	}
> +	preempt_enable();
> +	if (val >= 0) {
> +		down_read(&brw->rw_sem);
> +		atomic_inc(&brw->slow_read_ctr);
> +		up_read(&brw->rw_sem);
> +	} else {
> +		if (atomic_dec_and_test(&brw->slow_read_ctr))
> +			wake_up_all(&brw->write_waitq);
> +	}
> +}
> +EXPORT_SYMBOL(__percpu_down_up_read);
> +
> +static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
> +{
> +	unsigned int sum = 0;
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu) {
> +		sum += per_cpu(*brw->fast_read_ctr, cpu);
> +		per_cpu(*brw->fast_read_ctr, cpu) = 0;
> +	}
> +
> +	return sum;
> +}
> +
> +/*
> + * A writer takes ->writer_mutex to exclude other writers and to force the
> + * readers to switch to the slow mode, note the mutex_is_locked() check in
> + * update_fast_ctr().
> + *
> + * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
> + * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
> + * counter it represents the number of active readers.
> + *
> + * Finally the writer takes ->rw_sem for writing and blocks the new readers,
> + * then waits until the slow counter becomes zero.
> + */
> +void percpu_down_write(struct percpu_rw_semaphore *brw)
> +{
> +	/* also blocks update_fast_ctr() which checks mutex_is_locked() */
> +	mutex_lock(&brw->writer_mutex);
> +
> +	/*
> +	 * 1. Ensures mutex_is_locked() is visible to any down_read/up_read
> +	 *    so that update_fast_ctr() can't succeed.
> +	 *
> +	 * 2. Ensures we see the result of every previous this_cpu_add() in
> +	 *    update_fast_ctr().
> +	 *
> +	 * 3. Ensures that if any reader has exited its critical section via
> +	 *    fast-path, it executes a full memory barrier before we return.
> +	 */
> +	synchronize_sched();
> +
> +	/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
> +	atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
> +
> +	/* block the new readers completely */
> +	down_write(&brw->rw_sem);
> +
> +	/* wait for all readers to complete their percpu_up_read() */
> +	wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
> +}
> +EXPORT_SYMBOL(percpu_down_write);
> +
> +void percpu_up_write(struct percpu_rw_semaphore *brw)
> +{
> +	/* allow the new readers, but only the slow-path */
> +	up_write(&brw->rw_sem);
> +
> +	/* insert the barrier before the next fast-path in down_read */
> +	synchronize_sched();
> +
> +	mutex_unlock(&brw->writer_mutex);
> +}
> +EXPORT_SYMBOL(percpu_up_write);