Re: Thread flags modified without set_thread_flag() (non atomically)

Andrew Morton
2007-03-05T22:04:42Z

On Mon, 5 Mar 2007 15:40:33 +0100
Andi Kleen <ak@suse.de> wrote:

> > It does seem risky. Perhaps it is a micro-optimisation which utilises
> > knowledge that this thread_struct cannot be looked up via any path in this
> > context.
> >
> > Or perhaps it is a bug. Andi, can you please comment?
>
> On flush_thread nobody else can mess with the thread,

What about resched_task()?

> so yes it's a micro
> optimization.
>
> >
> > > And about this specific flush_thread, I am puzzled about the t->flags ^=
> > > (_TIF_ABI_PENDING | _TIF_IA32); line. The XOR will clearly flip the
> > > _TIF_ABI_PENDING bit to 0, and very likely set _TIF_IA32 to the opposite
> > > of its current value. Why does this change need to be written atomically
> > > (can other threads play with these flags ?) ?
> > >
> >
> > Don't know.
>
> iirc it came from DaveM originally. He just likes to write things in
> comp^wclever ways :0) It's just a little shorter.
>
> > No, I don't immediately see anything in the flush_old_exec() code path
> > which tells us that nobody else can look up this thread_info (or be holding
> > a ref to it) in this context.
>
> Normally the process flags atomicity should only matter with signals;

Thread flags. yes, most of them are synchronously set by their owner, but
not all, I think.

> i don't think you can send a signal to a process being in exec this way.
>
> -Andi