Re: Linus rollup

[Richard Henderson <rth@twiddle.net>]

On Wed, Jan 29, 2003 at 06:00:54PM -0800, Andrew Morton wrote:
>  * Expected reader usage:
>  * 	do {
>  *	    seq = fr_read_begin();
>  * 	...
>  *      } while (seq != fr_read_end());

I think perhaps

	do {
	  seq = fr_read_begin(&lock);
	  ...
	} while (fr_read_end(&lock, seq))

would be a better interface.  This would allow you to change
the implementation as well.  E.g.

	unsigned fr_read_begin(frlock_t *lock)
	{
	  unsigned s;

	  do
	    {
	      barrier ();
	      s = lock->sequence;
	    }
	  while (s & 1);
	  rmb();

	  return s;
	}

	int fr_read_end(frlock_t *lock, unsigned s)
	{
	  rmb();
	  return s == lock->sequence;
	}

	void fr_write_begin(frlock_t *rw)
	{
	  rw->sequence++;
	  wmb();
	}

	void fr_write_begin(frlock_t *rw)
	{
	  wmb();
	  rw->sequence++;
	}

which doesn't require as much memory.


r~
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