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

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Date:	Mon, 24 Nov 2014 12:34:35 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.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 12:04 PM, David Howells <dhowells@...hat.com> wrote:
>
> Reserve ACCESS_ONCE() for reading and add an ASSIGN_ONCE() or something like
> that for writing?

I wouldn't mind that. We've had situations where reading and writing
isn't really similar - like alpha where reading a byte is atomic, but
writing one isn't.

Then we could also make it have the "get_user()/put_user()" kind of
semantics - .and then use the same "sizeopf()" tricks that we use for
get_user/put_user.

That would actually work around the gcc bug a completely different way:

  #define ACCESS_ONCE(p) \
      ({ typeof(*p) __val; __read_once_size(p, &__val, sizeof(__val)); __val; })

and then we can do things like this:

  static __always_inline void __read_once_size(volatile void *p, void
*res, int size)
  {
       switch (size) {
       case 1: *(u8 *)res = *(volatile u8 *)p; break;
       case 2: *(u16 *)res = *(volatile u16 *)p; break;
       case 4: *(u32 *)res = *(volatile u32 *)p; break;
#ifdef CONFIG_64BIT
       case 8: *(u64 *)res = *(volatile u64 *)p; break;
#endif
       }
  }

and same for ASSIGN_ONCE(val, p).

That also hopefully avoids the whole "oops, gcc has a bug", because
the actual volatile access is always done using a scalar type, even if
the type of "__val" may in fact be a structure.

Christian, how painful would that be? Sorry to try to make you do a
totally different approach..

                  Linus
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