Re: [PATCH] tools: restore READ_ONCE() C++ compatibility

[Arnaldo Carvalho de Melo]

Em Mon, Apr 09, 2018 at 06:10:41PM +0100, Mark Rutland escreveu:
> Hi Arnaldo,
> 
> As Sandipan gave a Tested-by for this, are you happy to pick it up as a
> fix for v4.17?
> 
> Or would you prefer that I resend this?

I forgot about this fix, but was exposed to it while processing
Sandipan's patches for fixing up builtin clang support, so I ended up
adding the following patch:

https://git.kernel.org/acme/c/ad0902e0c400

This sidesteps this issue by removing the sequence of includes that ends
up including the compiler.h from a C++ file.

Now 'make LIBCLANGLLVM=1 -C tools/perf' works, but I'll look at the
patch below, probably it will save some time in the future if we get to
include compiler.h from C++ code again...

Take a look at my perf/urgent branch, that I just asked Ingo to pull.

- Arnaldo
 
> Thanks,
> Mark.
> 
> On Wed, Apr 04, 2018 at 05:34:45PM +0100, Mark Rutland wrote:
> > Our userspace <linux/compiler.h> defines READ_ONCE() in a way that clang
> > doesn't like, as we have an anonymous union in which neither field is
> > initialized.
> > 
> > WRITE_ONCE() is fine since it initializes the __val field. For
> > READ_ONCE() we can keep clang and GCC happy with a dummy initialization
> > of the __c field, so let's do that.
> > 
> > At the same time, let's split READ_ONCE() and WRITE_ONCE() over several
> > lines for legibility, as we do in the in-kernel <linux/compiler.h>.
> > 
> > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > Fixes: 6aa7de059173a986 ("locking/atomics: COCCINELLE/treewide: Convert trivial ACCESS_ONCE() patterns to READ_ONCE()/WRITE_ONCE()")
> > Reported-by: Li Zhijian <lizhijian@xxxxxxxxxxxxxx>
> > Reported-by: Sandipan Das <sandipan@xxxxxxxxxxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > ---
> >  tools/include/linux/compiler.h | 20 +++++++++++++++-----
> >  1 file changed, 15 insertions(+), 5 deletions(-)
> > 
> > Hi,
> > 
> > This is fallout from my automated ACCESS_ONCE() removal, and I'm not that
> > familiar with using clang for perf.
> > 
> > In local testing, this fixes READ_ONCE() when compiling with clang, but I
> > subsequently hit some other issues which I believe are down to LLVM API
> > changes.
> > 
> > Zhijian, Sandipan, does this patch work for you?
> > 
> > Thanks,
> > Mark.
> > 
> > diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
> > index 04e32f965ad7..1827c2f973f9 100644
> > --- a/tools/include/linux/compiler.h
> > +++ b/tools/include/linux/compiler.h
> > @@ -151,11 +151,21 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
> >   * required ordering.
> >   */
> >  
> > -#define READ_ONCE(x) \
> > -	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
> > -
> > -#define WRITE_ONCE(x, val) \
> > -	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
> > +#define READ_ONCE(x)					\
> > +({							\
> > +	union { typeof(x) __val; char __c[1]; } __u =	\
> > +		{ .__c = { 0 } };			\
> > +	__read_once_size(&(x), __u.__c, sizeof(x));	\
> > +	__u.__val;					\
> > +})
> > +
> > +#define WRITE_ONCE(x, val)				\
> > +({							\
> > +	union { typeof(x) __val; char __c[1]; } __u =	\
> > +		{ .__val = (val) }; 			\
> > +	__write_once_size(&(x), __u.__c, sizeof(x));	\
> > +	__u.__val;					\
> > +})
> >  
> >  
> >  #ifndef __fallthrough
> > -- 
> > 2.11.0
> >