Re: [PATCH] Fix x86_64 TIF_SYSCALL_TRACE race in entry.S

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Andi Kleen (andi@firstfloor.org) wrote:
> > Setting the thread flag being an atomic operation, I would expect
> > setting/clearing it asynchronously from another thread to be a valid
> 
> It could be a very short stop. Also do you start kernel tracing that often?
> 

It's not a matter of how often I start tracing, but more about what
impact I want this operation to have on a running production system. If
I start tracing on a server to try detecting particularly nasty race
conditions, I prefer not to interfere with the normal execution too
much. The same applies when we try to figure out the source of some
unexpected latencies experienced in user-space : stopping the processes
could be considered as having too much impact on the system studied.

I was already reluctant about iterating on every thread to set a flag
(this was proposed by Martin and Rebecca, in their Google ktrace
implementation), but I accepted to go forward this solution because of
the performance benefits. However, I would prefer not to go as far as
stopping each process on the system upon trace start/stop to perform
this unless it's the only solution left.

> > Here is a modified version where I add my test only in the path where we
> > know that we have work to do, therefore removing the supplementary test
> > from the performance critical path. Would it be more acceptable ?
> 
> It's better, but stopping would be even better. I wouldn't
> be surprised if there are other problems with async thread flags changing.
>  

Do you mean architectures other than x86_64 could also assume that the
thread flags will stay unchanged between two consecutive reads ? If
those thread flags were meant not to be asynchronously updated, why
would they require an atomic update at all ?


> Also I object to you calling this a bug. It's a new feature.
> 

Agreed. ptrace seems to be correct as is. It would only be needed if we
plan to use the flags as I described TIF_KERNEL_TRACE.

Mathieu


-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68