linux-kernel - Re: [PATCH v3] locking/qrwlock: Let qrwlock has same layout regardless of the endian

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Message-ID: <20160714093733.GF30909@twins.programming.kicks-ass.net>
Date:	Thu, 14 Jul 2016 11:37:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	mingo@...hat.com, arnd@...db.de, Waiman.Long@....com
Subject: Re: [PATCH v3] locking/qrwlock: Let qrwlock has same layout
 regardless of the endian

On Thu, Jul 14, 2016 at 03:44:42PM +0800, xinhui wrote:
> >OK, so I poked at this a bit and I ended up with the below; but now
> >qrwlock and qspinlock are inconsistent; although I suspect qspinlock is
> >similarly busted wrt endian muck.
> >
> >Not sure what to do..
> >
> Lets talk about the qspinlock.
> 
> for x86, We has already assumed that ->locked sit at the low 8 bits, as is
> smp_store_release((u8 *)lock, 0);

Right, true on x86 though :-) I noticed your PPC patches have a +3 in
there conditional on __BIG_ENDIAN.

> Then we can do a favor, export ->locked but other fields as reserved.
> say
> 
> struct __qspinlock_unlcok_interface {/* what name is better?*/
> #ifdef __LITTLE_ENDIAN
> 		u8      locked;
> 		u8      reserved[3]; /* do not touch it, internally use only  */
> #else
> 		u8      reserved[3];
> 		u8      locked;
> #endif
> };

Right, maybe, although something like:

static inline u8 *__qspinlock_lock_byte(struct qspinlock *lock)
{
	return (u8 *)lock + 3 * IS_BUILTIN(__BIG_ENDIAN);
}

static inline u8 *__qrwlock_write_byte(struct qrwlock *lock)
{
	return (u8 *)lock + 3 * IS_BUILTIN(__BIG_ENDIAN);
}

is shorter?


> >  /*
> >+ * Writer states & reader shift and bias.
> >+ *
> >+ *       | +0 | +1 | +2 | +3 |
> >+ *   ----+----+----+----+----+
> >+ *    LE | 12 | 34 | 56 | 78 | 0x12345678
> >+ *   ----+----+----+----+----+
> >+ *    BE | 78 | 56 | 34 | 12 | 0x12345678
> >+ *   ----+----+----+----+----+
> >+ *       | wr |      rd      |
> >+ *       +----+----+----+----+
> >+ *
> >   */
> 
> very clearly. :)

I did one for the qspinlock code too..

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index b2caec7315af..9191dc454e96 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -120,6 +120,23 @@ static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
  *
  * This internal structure is also used by the set_locked function which
  * is not restricted to _Q_PENDING_BITS == 8.
+ *
+ *       | +0 | +1 | +2 | +3 |
+ *   ----+----+----+----+----+
+ *    LE | 78 | 56 | 34 | 12 |	val = 0x12345678
+ *   ----+----+----+----+----+
+ *    LE | 34 | 12 |		locked_pending = 0x1234
+ *   ----+----+----+----+----+
+ *       | L  | P  |  tail   |
+ *       +----+----+----+----+
+ *
+ *   ----+----+----+----+----+
+ *    BE | 12 | 34 | 56 | 78 |	val = 0x12345678
+ *   ----+----+----+----+----+
+ *    BE           | 12 | 34 |	locked_pending = 0x1234
+ *   ----+----+----+----+----+
+ *       |  tail   | P  | L  |
+ *       +----+----+----+----+
  */
 struct __qspinlock {
 	union {