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Message-ID: <20160602110200.GZ3190@twins.programming.kicks-ass.net>
Date: Thu, 2 Jun 2016 13:02:00 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
waiman.long@...com
Subject: Re: [PATCH] locking/qrwlock: fix write unlock issue in big endian
On Thu, Jun 02, 2016 at 12:44:51PM +0200, Arnd Bergmann wrote:
> On Thursday, June 2, 2016 6:09:08 PM CEST Pan Xinhui wrote:
> > diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> > index 54a8e65..eadd7a3 100644
> > --- a/include/asm-generic/qrwlock.h
> > +++ b/include/asm-generic/qrwlock.h
> > @@ -139,7 +139,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
> > */
> > static inline void queued_write_unlock(struct qrwlock *lock)
> > {
> > - smp_store_release((u8 *)&lock->cnts, 0);
> > + (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
> > }
>
> Isn't this more expensive than the existing version?
Yes, loads. And while this might be a suitable fix for asm-generic, it
will introduce a fairly large regression on x86 (which is currently the
only user of this).
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