linux-kernel - Re: [PATCH] coredump: Replace opencoded set_mask

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Message-ID: <20150807154525.GO16853@twins.programming.kicks-ass.net>
Date:	Fri, 7 Aug 2015 17:45:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <vgupta@...opsys.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] coredump: Replace opencoded set_mask_bits()

On Fri, Aug 07, 2015 at 09:05:06PM +0530, Vineet Gupta wrote:
> On Friday 07 August 2015 08:27 PM, Peter Zijlstra wrote:
> > On Fri, Aug 07, 2015 at 08:14:03PM +0530, Vineet Gupta wrote:
> > 
> >> > See, I have such a cmpxchg loop in ARC code - originally from Peter :-)
> >> > arch/arc/kernel/smp.c. @ipi_data_ptr is NOT atomic_t
> >> > 
> >> > 	do {
> >> > 		new = old = ACCESS_ONCE(*ipi_data_ptr);
> >> > 		new |= 1U << msg;
> >> > 	} while (cmpxchg(ipi_data_ptr, old, new) != old);
> >> > 
> > Well, you'll have atomic_or() real soon now.
> 
> Doesn't help my cause - ipi_data_ptr is not atomic_t - hence my prev question in
> this thread

A cast will work :-)

But yes, ideally everything will be type safe because of those archs
that cannot have atomic RmW ops like !ARC_HAS_LLSC.

Mixing cmpxchg()/xchg() with regular stores is broken on those.

Fwiw, you might want to set ARCH_SUPPORTS_ATOMIC_RMW for ARC_HAS_LLSC.
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