linux-kernel - Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

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Date:	31 Jan 2014 05:03:48 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	peterz@...radead.org, Waiman.Long@...com
Cc:	akpm@...ux-foundation.org, andi@...stfloor.org, arnd@...db.de,
	aswin@...com, hpa@...or.com, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux@...izon.com, mingo@...hat.com,
	paulmck@...ux.vnet.ibm.com, raghavendra.kt@...ux.vnet.ibm.com,
	riel@...hat.com, rostedt@...dmis.org, scott.norton@...com,
	tglx@...utronix.de, tim.c.chen@...ux.intel.com,
	torvalds@...ux-foundation.org, walken@...gle.com, x86@...nel.org
Subject: Re: [PATCH v11 0/4] Introducing a queue read/write lock implementation

How about getting rid of that TICKET_MSB mess and doing something like:

#define TICKET_MASK	0xFFFF

static inline void ticket_spin_unlock(atomic_t *tickets)
{
	u32 t = *tickets;

	smp_mb__before_atomic_inc();

	/* Increment the low 16 bits without affecting the upper. */
	if (unlikely((~t & TICKET_MASK) == 0))
		atomic_add(-(atomic_t)TICKET_MASK, tickets);
	else
		atomic_inc(tickets);
}

That also allows up to 2^16 waiters, up from 2^15.
(Minus one on both cases, if you want to be fussy.)
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