Re: [PATCH -mm]percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.fix

[Paul E. McKenney]

On Sun, Nov 11, 2012 at 07:27:44PM +0100, Oleg Nesterov wrote:
> More include's and more comments, no changes in code.
> 
> To remind, once/if I am sure you agree with this patch I'll send 2 additional
> and simple patches:
> 
> 	1. lockdep annotations
> 
> 	2. CONFIG_PERCPU_RWSEM
> 
> It seems that we can do much more improvements to a) speedup the writers and
> b) make percpu_rw_semaphore more useful, but not right now.
> 
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Looks good to me!

							Thanx, Paul

> ---
>  lib/percpu-rwsem.c |   35 +++++++++++++++++++++++++++++++++--
>  1 files changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c
> index 0e3bc0f..02bd157 100644
> --- a/lib/percpu-rwsem.c
> +++ b/lib/percpu-rwsem.c
> @@ -1,6 +1,11 @@
> +#include <linux/mutex.h>
> +#include <linux/rwsem.h>
> +#include <linux/percpu.h>
> +#include <linux/wait.h>
>  #include <linux/percpu-rwsem.h>
>  #include <linux/rcupdate.h>
>  #include <linux/sched.h>
> +#include <linux/errno.h>
> 
>  int percpu_init_rwsem(struct percpu_rw_semaphore *brw)
>  {
> @@ -21,6 +26,29 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
>  	brw->fast_read_ctr = NULL; /* catch use after free bugs */
>  }
> 
> +/*
> + * This is the fast-path for down_read/up_read, it only needs to ensure
> + * there is no pending writer (!mutex_is_locked() check) and inc/dec the
> + * fast per-cpu counter. The writer uses synchronize_sched() to serialize
> + * with the preempt-disabled section below.
> + *
> + * The nontrivial part is that we should guarantee acquire/release semantics
> + * in case when
> + *
> + *	R_W: down_write() comes after up_read(), the writer should see all
> + *	     changes done by the reader
> + * or
> + *	W_R: down_read() comes after up_write(), the reader should see all
> + *	     changes done by the writer
> + *
> + * If this helper fails the callers rely on the normal rw_semaphore and
> + * atomic_dec_and_test(), so in this case we have the necessary barriers.
> + *
> + * But if it succeeds we do not have any barriers, mutex_is_locked() or
> + * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
> + * reader inside the critical section. See the comments in down_write and
> + * up_write below.
> + */
>  static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
>  {
>  	bool success = false;
> @@ -98,6 +126,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw)
>  	 *
>  	 * 3. Ensures that if any reader has exited its critical section via
>  	 *    fast-path, it executes a full memory barrier before we return.
> +	 *    See R_W case in the comment above update_fast_ctr().
>  	 */
>  	synchronize_sched();
> 
> @@ -116,8 +145,10 @@ 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 */
> +	/*
> +	 * Insert the barrier before the next fast-path in down_read,
> +	 * see W_R case in the comment above update_fast_ctr().
> +	 */
>  	synchronize_sched();
> -
>  	mutex_unlock(&brw->writer_mutex);
>  }
> -- 
> 1.5.5.1
> 
> 

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