linux-kernel - Re: [PATCH] m32r: Revise __raw_read

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Date:	Fri, 22 Sep 2006 05:27:08 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Hirokazu Takata <takata@...ux-m32r.org>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] m32r: Revise __raw_read_trylock()

On Fri, Sep 22, 2006 at 04:48:13PM +0900, Paul Mundt wrote:
> This might be a stupid question, but why exactly are we ripping out
> generic__raw_read_trylock() if architectures are going to implement a
> generic implementation anyways, rather than just changing it to match
> the proper semantics?

Because there is no generic definition of struct spinlock.

>  int __lockfunc generic__raw_read_trylock(raw_rwlock_t *lock)
>  {
> -	__raw_read_lock(lock);
> -	return 1;
> +	atomic_t *count = (atomic_t *)lock;
> +	atomic_dec(count);
> +	if (atomic_read(count) >= 0)
> +		return 1;
> +	atomic_inc(count);
> +	return 0;
>  }

You're assuming:

 - a spinlock is an atomic_t.
 - Said atomic_t uses RW_LOCK_BIAS to indicate locked/unlocked.

This is true for m32r, but not for sparc.  SuperH looks completely
broken -- I don't see how holding a read lock prevents someone else from
getting a write lock.  The SH write_trylock uses RW_LOCK_BIAS, but
write_lock doesn't.  Are there any SMP SH machines?
-
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