On Tue, 20 Mar 2007, Oleg Nesterov wrote:

> On 03/20, Oleg Nesterov wrote:
> >
> > On 03/19, Davide Libenzi wrote:
> > >
> > > I'd need a get_task_struct in any case in order to safely call 
> > > unlock_task_sighand(). At that point I'd prefer to just pass through the 
> > > struct pid*. I'll be posting the new version for review as soon as I 
> > > complete a few tests ...
> > 
> > If signalfd_get_sighand()->lock_task_sighand() succeeds, it is safe to
> > dereference ctx->tsk. The task can't be freed and ctx->tsk can't be cleared
> > while we are holding siglock.
> > 
> > However, I was wrong, we still need a re-check after lock_task_sighand().
> > We should check ctx->tsk != NULL.
> 
> IOW, we can (afaics) do
> 
>       static struct sighand_struct *signalfd_get_sighand(struct signalfd_ctx 
> *ctx,
>                                                          unsigned long *flags)
>       {
>               struct sighand_struct *sighand = NULL;
>               struct tsak_struct *tsk;
> 
>               rcu_read_lock();
>               tsk = rcu_dereference(ctx->tsk);  // not needed, just a 
> documentation
>               if (tsk != NULL)
>                       sighand = lock_task_sighand(tsk, flags);
>               rcu_read_unlock();
> 
>               if (sighand && !ctx->tsk)) {
>                       unlock_task_sighand(tsk, flags);
>                       sighand = NULL;
>               }
> 
>               return sighand;
>       }
> 
> If signalfd_get_sighand() succeeds, ctx->tsk is pinned.

I did a similar thing, but I renamed the locking functions and its 
parameters. After looking at what the pid thing was doing, I realized that 
it was not really needed.



- Davide


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