linux-kernel - Re: [PATCH 08/10] rwsem: down_read

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Date:	Wed, 19 May 2010 14:34:15 +0100
From:	David Howells <dhowells@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 08/10] rwsem: down_read_critical infrastructure support

Michel Lespinasse <walken@...gle.com> wrote:

>  #define RWSEM_WAITING_FOR_READ	0x00000001
>  #define RWSEM_WAITING_FOR_WRITE	0x00000002
> +#define RWSEM_UNFAIR            0x00000004

Can I suggest you change this to:

	enum rwsem_waiter_type {
		RWSEM_WAITING_FOR_WRITE,
		RWSEM_WAITING_FOR_READ,
		RWSEM_WAITING_FOR_UNFAIR_READ
	};

and then change:

>  	unsigned int flags;

to:

	enum rwsem_waiter_type type;

and use this throughout.  It simplifies some of the code too.  See attached
patch.

David
---
rwsem: Make waiter type an enum in the slow path of the asm-optimised version

Make the waiter type in the asm-optimised version of the slow path an
enumeration rather than a bitmask.

Signed-off-by: David Howells <dhowells@...hat.com>
---
diff --git a/lib/rwsem.c b/lib/rwsem.c
index b3fe179..8aa2238 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -28,13 +28,16 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
 
 EXPORT_SYMBOL(__init_rwsem);
 
+enum rwsem_waiter_type {
+	RWSEM_WAITING_FOR_WRITE,
+	RWSEM_WAITING_FOR_READ,
+	RWSEM_WAITING_FOR_UNFAIR_READ
+};
+
 struct rwsem_waiter {
 	struct list_head list;
 	struct task_struct *task;
-	unsigned int flags;
-#define RWSEM_WAITING_FOR_READ	0x00000001
-#define RWSEM_WAITING_FOR_WRITE	0x00000002
-#define RWSEM_UNFAIR            0x00000004
+	enum rwsem_waiter_type type;
 };
 
 /* Wake types for __rwsem_do_wake().  Note that RWSEM_WAKE_NO_ACTIVE and
@@ -64,7 +67,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 	signed long oldcount, woken, loop, adjustment;
 
 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
-	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
+	if (waiter->type != RWSEM_WAITING_FOR_WRITE)
 		goto readers_only;
 
 	if (wake_type == RWSEM_WAKE_READ_OWNED)
@@ -133,10 +136,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 		waiter = list_entry(waiter->list.next,
 					struct rwsem_waiter, list);
 
-	} while (waiter->flags & RWSEM_WAITING_FOR_READ);
+	} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
 
 	adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
-	if (waiter->flags & RWSEM_WAITING_FOR_READ)
+	if (waiter->type != RWSEM_WAITING_FOR_WRITE)
 		/* hit end of list above */
 		adjustment -= RWSEM_WAITING_BIAS;
 
@@ -172,7 +175,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
  */
 static struct rw_semaphore __sched *
 rwsem_down_failed_common(struct rw_semaphore *sem,
-			 unsigned int flags, signed long adjustment)
+			 enum rwsem_waiter_type type, signed long adjustment)
 {
 	struct rwsem_waiter waiter;
 	struct task_struct *tsk = current;
@@ -183,13 +186,13 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 	/* set up my own style of waitqueue */
 	spin_lock_irq(&sem->wait_lock);
 	waiter.task = tsk;
-	waiter.flags = flags;
+	waiter.type = type;
 	get_task_struct(tsk);
 
 	if (list_empty(&sem->wait_list))
 		adjustment += RWSEM_WAITING_BIAS;
 
-	if (flags & RWSEM_UNFAIR)
+	if (type & RWSEM_WAITING_FOR_UNFAIR_READ)
 		list_add(&waiter.list, &sem->wait_list);
 	else
 		list_add_tail(&waiter.list, &sem->wait_list);
@@ -242,8 +245,7 @@ rwsem_down_read_failed(struct rw_semaphore *sem)
 asmregparm struct rw_semaphore __sched *
 rwsem_down_read_unfair_failed(struct rw_semaphore *sem)
 {
-	return rwsem_down_failed_common(sem,
-					RWSEM_WAITING_FOR_READ | RWSEM_UNFAIR,
+	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_UNFAIR_READ,
 					-RWSEM_ACTIVE_READ_BIAS);
 }
 
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