linux-kernel - Re: futex question

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Date:	Fri, 02 Oct 2009 16:38:59 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Anirban Sinha <ASinha@...gmasystems.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: futex question

Anirban Sinha wrote:
>> Should we not just clear the pointer (and
>>> it's compat version) within do_execve()?
> 
> 
> In our private repository, applying the following patch resolved the
> issues I mentioned. I no longer see messages like this:
> 
>  [futex] ("ifconfig")(pid=2509) exit_robust_list:unable to fetch robust
> entry. uaddr=0x000000002abbc4f0
> 
> from my instrumented kernel within exit_robust_list(). My
> instrumentation looked something like this:

> 
> if (fetch_robust_entry(...)) {
> printk(...);
> return;
> }
> 
> Just tossing the patch in the community in case someone is interested


Thanks for sending the patch.  I'm looking into it now.  Couple questions:

1) What caused you to instrument this path in the first place?  Were you 
seeing some unexpected behavior?

2) I wonder why we would need to clear the robust list, but I don't see 
other things like pi_blocked_on, etc. in execve being cleared.  I'm 
looking into this now (perhaps we don't do the same cleanup, need to 
check).... have to get on the plane...

--
Darren Hart

> ...
> 
> 
> 
> Signed-off-by: Anirban Sinha <asinha@...gmasystems.com>
> ---
>  fs/compat.c |    3 +++
>  fs/exec.c   |    3 +++
>  2 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/compat.c b/fs/compat.c
> index 6d6f98f..c3d117c 100644
> --- a/fs/compat.c
> +++ b/fs/compat.c
> @@ -1510,6 +1510,9 @@ int compat_do_execve(char * filename,
>         if (retval)
>                 goto out_file;
> 
> +#ifdef CONFIG_FUTEX
> +       current->compat_robust_list = NULL;
> +#endif
>         bprm->argc = compat_count(argv, MAX_ARG_STRINGS);
>         if ((retval = bprm->argc) < 0)
>                 goto out;
> diff --git a/fs/exec.c b/fs/exec.c
> index 172ceb6..e9334b8 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1323,6 +1323,9 @@ int do_execve(char * filename,
>         retval = bprm_mm_init(bprm);
>         if (retval)
>                 goto out_file;
> +#ifdef CONFIG_FUTEX
> +       current->robust_list = NULL;
> +#endif
> 
>         bprm->argc = count(argv, MAX_ARG_STRINGS);
>         if ((retval = bprm->argc) < 0)
> 
> 


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
--
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