linux-kernel - Re: [PATCH/RFC 7/7] kernel: Force ACCESS

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Date:	Mon, 24 Nov 2014 13:02:40 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	David Howells <dhowells@...hat.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-mips <linux-mips@...ux-mips.org>,
	linux-x86_64@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

On Mon, Nov 24, 2014 at 12:53 PM, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
>
> That looks like a lot of changes all over ACCESS_ONCE -> ASSIGN_ONCE:
> git grep "ACCESS_ONCE.*=.*"
> gives me 200 placea not in Documentation.

Yeah, that's a bit annoying.

How about a combination of the two:

 - accept the fact that right now ACCESS_ONCE() is fairly widespread
(even for writing)

 - but also admit that we'd be better off with a nicer interface

and make the solution be:

 - make ACCESS_ONCE() only work on scalars, and deprecate it

 - add new "read_once()" and "write_once()" interfaces that *do* work
on (appropriately sized) structures and unions, and start migrating
things over. In particular, start with the ones that can no longer use
ACCESS_ONCE() because they aren't scalar..

That second point would make the conversion patches actually easier to
read. Instead of this:

 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-       struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
+       arch_spinlock_t tmp = {};

-       return tmp.tail != tmp.head;
+       tmp.head_tail =ACCESS_ONCE(lock->head_tail);
+       return tmp.tickets.tail != tmp.tickets.head;
 }

which isn't *complex*, but is also not an obvious conversion, we'd have just

 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-       struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
-       struct __raw_tickets tmp = read_once(lock->tickets);

        return tmp.tail != tmp.head;
 }

which is a much simpler and more obvious change.

And then we could slowly try to migrate existing ACCESS_ONCE() users
over (particularly writers).

Hmm? Too much?

                     Linus
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