linux-kernel - Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless accesses with ACCESS

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Date:	Fri, 09 Jan 2015 22:58:50 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	Davidlohr Bueso <dave@...olabs.net>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com,
	Pranith Kumar <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless
 accesses with ACCESS_ONCE()

Am 09.01.2015 um 14:56 schrieb Peter Zijlstra:
> On Fri, Jan 09, 2015 at 05:49:54AM -0800, Paul E. McKenney wrote:
>>> That reminds me, I think the new conversion for stores will most likely
>>> introduce silly arg bugs:
>>>
>>> -       ACCESS_ONCE(a) = b;
>>> +       ASSIGN_ONCE(b, a);
>>
>> I was planning to do mine by hand for this sort of reason.
>>
>> Or are you thinking of something more subtle than the case where
>> "b" is an unparenthesized comma-separated expression?
> 
> I think he's revering to the wrong way around-ness of the thing.
> 
> Its a bit of a mixed bag on assignments, but for instance
> rcu_assign_pointer() takes them the right way around, as does
> atomic_set().
> 
> So yes, I think the ASSIGN_ONCE() thing got the arguments the wrong way
> around.
> 
> We could maybe still change it, before its in too long ?

Linus initial proposal was inspired by put_user model which is (val, ptr) and I took that.
As my focus was on avoiding the volatile bug, all my current conversions are READ_ONCE as no potential ASSIGN_ONCE user was done on a non-scalar type, so I have no first hand experience. I am fine with changing that, though, both ways have pros and cons. Last time I checked in Linus tree there was no ASSIGN_ONCE user.

When we talk about changing the parameters it might make sense to also think about some comments from George Spelvin and consider a rename to WRITE_ONCE or STORE_ONCE (READ_ONCE --> LOAD_ONCE). Unfortunately there doesnt seem to be a variant that is fool proof (in the sense of Rustys guideline that a good interface cannot be used wrong). So any proposal in that regard would be very welcome.

Christian

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