On 06/03, Linus Torvalds wrote: > > On Tue, Jun 3, 2014 at 10:26 AM, Oleg Nesterov <o...@redhat.com> wrote: > > > > looks like, SLAB_DESTROY_BY_RCU logic is broken? > > I haven't looked at the code yet, but SLAB_DESTROY_BY_RCU can be > subtle and very dangerous. > > The danger is that the *slab* itself is free'd by RCU, but individual > allocations can (and do) get re-used FOR THE SAME OBJECT TYPE without > waiting for RCU! > > This is subtle. It means that most people who think that "it's free'd > by RCU" get it wrong. Because individual allocations really aren't at > all RCU-free'd, it's just that the underlying memory is guaranteed to > not change type or be entirely thrown away until after a RCU grace > period.
Yes, exactly. And unless you use current->sighand (which is obviously stable) you need lock_task_sighand() which relies on ->siglock initialized by sighand_ctor(). > Without looking at the code, it sounds like somebody may doing things > to "sighand->lock->wait_list" that they shouldn't do. We've had cases > like that before, and most of them have been changed to *not* use > SLAB_DESTROY_BY_RCU, and instead make each individual allocation be > RCU-free'd (which is a lot simpler to think about, because then you > don't have the whole re-use issue). Sure, we only need to change __cleanup_sighand() to use call_rcu(). But I am not sure this makes sense, I mean, I do not think this can make something more simple/clear. > And this could easily be an RT issue, if the RT code does some > re-initialization of the rtmutex that replaces the spinlock we have. Unlikely... this should be done by sighand_ctor() anyway. I'll try to recheck rt_mutex_unlock() tomorrow. _Perhaps_ rcu_read_unlock() should be shifted from lock_task_sighand() to unlock_task_sighand() to ensure that rt_mutex_unlock() does nothihg with this memory after it makes another lock/unlock possible. But if we need this (currently I do not think so), this doesn't depend on SLAB_DESTROY_BY_RCU. And, at first glance, in this case rcu_read_unlock_special() might be wrong too. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
