Re: [PATCH] Async signal safe TLS accesses

[OndÅej BÃlka <neleai at seznam dot cz>]

On Thu, Dec 05, 2013 at 02:58:11PM -0800, Andrew Hunter wrote:
> On Thu, Dec 5, 2013 at 2:47 PM, OndÅej BÃlka <neleai@seznam.cz> wrote:
> > On Thu, Dec 05, 2013 at 12:01:11PM -0800, Andrew Hunter wrote:
> >>     TLS accesses from initial-exec variables are async-signal-safe.  Even
> >> +
> >> +  size_t pg = GLRO (dl_pagesize);
> >> +  size_t padded_size;
> >> +  if (boundary <= pg)
> >> +    {
> >> +      /* We'll get a pointer certainly aligned to boundary, so just
> >> +      add one more boundary-sized chunk to hold the header.  */
> >> +      padded_size = roundup (size, boundary) + boundary;
> >> +    }
> >> +  else
> >> +    {
> >> +      /* If we want K pages aligned to a J-page boundary, K+J+1 pages
> >> +      contains at least one such region that isn't directly at the start
> >> +      (so we can place the header.)  This is wasteful, but you're the one
> >> +      who wanted 64K-aligned TLS.  */
> >> +      padded_size = roundup (size, pg) + boundary + pg;
> >> +    }
> >> +
> > Is special casing here really needed?
> >
> 
> shrug, "needed" is hard to say, but given that it's written already I
> don't see a reason to actively exclude it.  I don't think it
> dramatically complicates things.
> 
> >
> > snip
> >
> >> +      /* busywork */
> >> +      free (malloc (128));
> >
> > that gets optimized away.
> >
> 
> It does not:
> 0000000000401780 <spin>:
>   401780:       48 83 ec 08             sub    $0x8,%rsp
>   401784:       0f 1f 40 00             nopl   0x0(%rax)
>   401788:       bf 80 00 00 00          mov    $0x80,%edi
>   40178d:       e8 5e f9 ff ff          callq  4010f0 <malloc@plt>
>   401792:       48 89 c7                mov    %rax,%rdi
>   401795:       e8 06 f8 ff ff          callq  400fa0 <free@plt>
>   40179a:       eb ec                   jmp    401788 <spin+0x8>
>   40179c:       0f 1f 40 00             nopl   0x0(%rax)
> 
> 
> It could be in principle and perhaps one should disable builtin malloc
> in Makefile, but this actually works as written with my gcc (4.6.3).
> 
Then you have old gcc. On debian 4.7.3 this get optimized away.