On Tue, Oct 08, 2019 at 05:26:59PM +0200, Andrea Parri wrote:
On Tue, Oct 08, 2019 at 04:24:14PM +0200, Christian Brauner wrote:
On Tue, Oct 08, 2019 at 04:20:35PM +0200, Andrea Parri wrote:
On Mon, Oct 07, 2019 at 04:18:26PM +0200, Dmitry Vyukov wrote:
On Mon, Oct 7, 2019 at 4:14 PM Andrea Parri parri.andrea@gmail.com wrote:
> > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > > { > > struct signal_struct *sig = tsk->signal; > > - struct taskstats *stats; > > + struct taskstats *stats_new, *stats; > > > > - if (sig->stats || thread_group_empty(tsk)) > > - goto ret; > > + /* Pairs with smp_store_release() below. */ > > + stats = READ_ONCE(sig->stats); > > This pairing suggests that the READ_ONCE() is heading an address > dependency, but I fail to identify it: what is the target memory > access of such a (putative) dependency?
I would assume callers of this function access *stats. So the dependency is between loading stats and accessing *stats.
AFAICT, the only caller of the function in 5.4-rc2 is taskstats_exit(), which 'casts' the return value to a boolean (so I really don't see how any address dependency could be carried over/relied upon here).
This does not make sense.
But later taskstats_exit does:
memcpy(stats, tsk->signal->stats, sizeof(*stats));
Perhaps it's supposed to use stats returned by taskstats_tgid_alloc?
Seems reasonable to me. If so, replacing the READ_ONCE() in question with an smp_load_acquire() might be the solution. Thoughts?
I've done that already in my tree yesterday. I can resend for another review if you'd prefer.
Oh nice! No need to resend of course. ;D FWIW, I can check it if you let me know the particular branch/commit (guessing that's somewhere in git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git, yes?).
Oh ups, yeah of course :) https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=tas...
Thanks! Christian