Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Jan 19, 2014 at 11:56:02AM -0800, Linus Torvalds wrote:
> On Sun, Jan 19, 2014 at 12:04 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> >
> > OK, another approach would be to never add "select ARCH_USE_QUEUE_RWLOCK"
> > on Alpha, at least if the queued rwlocks really do want to atomically
> > manipulate bytes.  After all, the Alpha systems that I know about don't
> > have enough CPUs to make queued rwlocks necessary anyway.
> >
> > Much simpler solution!
> >
> > Is this what you were getting at, or am I missing your point?
> 
> You're missing something.
> 
> Just make the "writer" field be an "int" on little-endian archiectures
> (like alpha).
> 
> There is no reason for that field to be a "char" to begin with, as far
> as I can tell, since the padding of the structure means that it
> doesn't save any space. But even if that wasn't true, we could make an
> arch-specific type for "minimum type for locking".

On 64-bit systems (which includes Alpha), agreed, the field can be a
32-bit portion of a 64-bit structure that is then manipulated atomically.
Many 32-bit systems need the reader and writer counts to fix in 32 bits
in order to allow things like queue_read_trylock() to correctly account
for both readers and writers.

If there was a 32-bit system running Linux that did not support byte
accesses, there would be a problem, but I don't know of any such system.

> So my *point* was that it should be easy enough to just make sure that
> any data structures used for locking have types that are appropriate
> for that locking.

So something like the following for the qrwlock definition, with
appropriate C-preprocessor wrappers for the atomic-add accesses?

							Thanx, Paul

------------------------------------------------------------------------

typedef struct qrwlock {
	union qrwcnts {
#ifdef CONFIG_64B
		struct (
			int writer;
			int reader;
		};
		atomic_long_t rwa;
		u64	 rwc;
#else
		struct {
#ifdef __LITTLE_ENDIAN
			u8  writer;	/* Writer state		*/
#else
			u16 r16;	/* Reader count - msb	*/
			u8  r8;		/* Reader count - lsb	*/
			u8  writer;	/* Writer state		*/
#endif
		};
		atomic_t    rwa;	/* Reader/writer atomic	*/
		u32	    rwc;	/* Reader/writer counts */
	} cnts;
#endif
	struct mcs_spinlock *waitq;	/* Tail of waiting queue */
} arch_rwlock_t;