Re: [LTP] [PATCH v1] syscalls/signal06: add volatile to loop variable

[Petr Vorel <pvorel@suse.cz>]

> Hi!
> > Thanks! The bug was closed as 'adding "cx" worked'. Reading topperc's comment it
> > looks like there no other way to fix the issue on clang than workaround with
> > volatile. Does it mean that it's a syscall problem and clang can do nothing
> > about it?

> It's problem with the inline assembly in the body of the while loop, the
> call to the syscall changes the register value that is used for the D
> variable in the case of clang, so the loop exits prematurely. We have to
> add cx register to the clobber list for that asm statement so that
> compiler knows that it's changed by the assembly.

> Interfacing assembly with C is a bit tricky since you have to explain
> to compiler which registers are changed from the assembly otherwise the
> results are undefined.

> The patch should look like:

> diff --git a/testcases/kernel/syscalls/signal/signal06.c b/testcases/kernel/syscalls/signal/signal06.c
> index 64f886ee3..78efd0fb9 100644
> --- a/testcases/kernel/syscalls/signal/signal06.c
> +++ b/testcases/kernel/syscalls/signal/signal06.c
> @@ -73,7 +73,7 @@ void test(void)
>                 /* sys_tkill(pid, SIGHUP); asm to avoid save/reload
>                  * fp regs around c call */
>                 asm ("" : : "a"(__NR_tkill), "D"(pid), "S"(SIGHUP));
> -               asm ("syscall" : : : "ax");
> +               asm ("syscall" : : : "ax", "cx");

OK, I completely overlooked asm() in the test. Thanks!

Petr

>                 loop++;
>         }

> Although it may not be a complete as the llwm issue suggests we should
> have a look at calling conventions for the syscall and check if we need
> to add any other registers to the clobber list.

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