This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH 08/08] nptl: arm: Fix Race conditions in pthread cancellation (BZ#12683)
- From: Alexander Monakov <amonakov at ispras dot ru>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: Phil Blundell <pb at pbcl dot net>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 3 Sep 2015 20:48:28 +0300 (MSK)
- Subject: Re: [PATCH 08/08] nptl: arm: Fix Race conditions in pthread cancellation (BZ#12683)
- Authentication-results: sourceware.org; auth=none
- References: <55E4C300 dot 9080800 at linaro dot org> <1441148926 dot 1680 dot 51 dot camel at pbcl dot net> <55E768E8 dot 8020904 at linaro dot org> <1441276001 dot 22688 dot 18 dot camel at pbcl dot net> <55E86CC9 dot 9040507 at linaro dot org> <1441297157 dot 22688 dot 26 dot camel at pbcl dot net> <55E883A4 dot 1030705 at linaro dot org>
On Thu, 3 Sep 2015, Adhemerval Zanella wrote:
> On 03-09-2015 13:19, Phil Blundell wrote:
> > On Thu, 2015-09-03 at 12:52 -0300, Adhemerval Zanella wrote:
> >> I can change to:
> >>
> >> mov lr, pc
> >> b __syscall_do_cancel
> >>
> >> Which explicit state it is a tail cail that do not return.
> >
> > If you set lr like that then the implication is that it will return
> > (since otherwise lr would be unnecessary). A regular tail call would
> > just do the branch without changing lr at all.
>
> My first approach was to use compiler output to check how it handle tail
> call for ARM. For instance, the code:
>
> --
>
> void __do_cancel () __attribute__ ((noreturn));
>
> long int foo (long int nr);
>
> long int __syscall (long int *d, long int nr)
> {
> if (*d & 0x1)
> __do_cancel ();
>
> return foo (nr);
> }
>
> --
>
> generates the 'bl' for __do_cancel with both GCC 4.9 and GCC 6.0.
To provide a bit of (potentially non-obvious) context: GCC deliberately
suppresses tail call optimization when the call target is a noreturn function,
with the intent of providing full backtraces if the noreturn function
terminates the program.
In the example above, call to 'foo' would be tail-call-optimized, while the
call to '__do_cancel' wouldn't.
Alexander