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SubjectRe: [PATCH 08/10] rwsem: down_read_critical infrastructure support
Date
Michel Lespinasse <walken@google.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@redhat.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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