[patch] remove redundant kioctx->users ref count

[patch] remove redundant kioctx->users ref count

[Chen, Kenneth W]

In the ioctx destroy path, both exit_aio and io_destroy calls
wait_for_all_aios().  And in that function, it won't return until
there are no outstanding kiocb, tracked by ctx->reqs_active.  So
the ref counting on kioctx for every individual kiocb is overly
excessive.  We know we won't perform last put_ioctx when releasing
Kiocb. This should clear out the cache line conflict mentioned in
earlier post.


Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>


diff -Nurp linux-2.6.18/fs/aio.c linux-2.6.18.ken/fs/aio.c
--- linux-2.6.18/fs/aio.c	2006-09-19 20:42:06.000000000 -0700
+++ linux-2.6.18.ken/fs/aio.c	2006-10-12 13:33:09.000000000 -0700
@@ -423,7 +422,6 @@ static struct kiocb fastcall *__aio_get_
 	ring = kmap_atomic(ctx->ring_info.ring_pages[0], KM_USER0);
 	if (ctx->reqs_active < aio_ring_avail(&ctx->ring_info, ring)) {
 		list_add(&req->ki_list, &ctx->active_reqs);
-		get_ioctx(ctx);
 		ctx->reqs_active++;
 		okay = 1;
 	}
@@ -534,8 +532,6 @@ int fastcall aio_put_req(struct kiocb *r
 	spin_lock_irq(&ctx->ctx_lock);
 	ret = __aio_put_req(ctx, req);
 	spin_unlock_irq(&ctx->ctx_lock);
-	if (ret)
-		put_ioctx(ctx);
 	return ret;
 }
 
@@ -791,8 +787,7 @@ static int __aio_run_iocbs(struct kioctx
 		 */
 		iocb->ki_users++;       /* grab extra reference */
 		aio_run_iocb(iocb);
-		if (__aio_put_req(ctx, iocb))  /* drop extra ref */
-			put_ioctx(ctx);
+		__aio_put_req(ctx, iocb);
  	}
 	if (!list_empty(&ctx->run_list))
 		return 1;
@@ -1015,9 +1010,6 @@ put_rq:
 	if (waitqueue_active(&ctx->wait))
 		wake_up(&ctx->wait);
 
-	if (ret)
-		put_ioctx(ctx);
-
 	return ret;
 }
 

Re: [patch] remove redundant kioctx->users ref count

[Benjamin LaHaise]

On Thu, Oct 12, 2006 at 04:54:46PM -0700, Chen, Kenneth W wrote:
> @@ -1015,9 +1010,6 @@ put_rq:
>  	if (waitqueue_active(&ctx->wait))
>  		wake_up(&ctx->wait);
>  
> -	if (ret)
> -		put_ioctx(ctx);
> -
>  	return ret;
>  }

This part makes me worry -- at this point we no longer have anything 
pinning the ioctx, yet we touch ->wait after dropping the lock.  The 
only way around this is if rcu is introduced in the final free of an 
ioctx to ensure the structure remains around for a sufficient grace 
period.

		-ben
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