On Tue, Aug 21, 2012 at 09:24:32AM -0700, Paul E. McKenney wrote: > On Tue, Aug 21, 2012 at 05:20:11PM +0200, Peter Zijlstra wrote: > > On Tue, 2012-08-21 at 09:47 -0300, Rafael Aquini wrote: > > > + mapping = rcu_access_pointer(page->mapping); > > > + if (mapping) > > > + mapping = mapping->assoc_mapping; > > > > The comment near rcu_access_pointer() explicitly says: > > > > * Return the value of the specified RCU-protected pointer, but omit the > > * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful > > * when the value of this pointer is accessed, but the pointer is not > > * dereferenced, > > > > Yet you dereference the pointer... smells like fail to me. > > Indeed! > > This will break DEC Alpha. In addition, if ->mapping can transition > from non-NULL to NULL, and if you used rcu_access_pointer() rather > than rcu_dereference() to avoid lockdep-RCU from yelling at you about > not either being in an RCU read-side critical section or holding an > update-side lock, you can see failures as follows: > > 1. CPU 0 runs the above code, picks up mapping, and finds it non-NULL. > > 2. CPU 0 is preempted or otherwise delayed. (Keep in mind that > even disabling interrupts in a guest OS does not prevent the > host hypervisor from preempting!) > > 3. Some other CPU NULLs page->mapping. Because CPU 0 isn't doing > anything to prevent it, this other CPU frees the memory. > > 4. CPU 0 resumes, and then accesses what is now the freelist. > Arbitrarily bad things start happening. > > If you are in a read-side critical section, use rcu_dereference() instead > of rcu_access_pointer(). If you are holding an update-side lock, use > rcu_dereference_protected() and say what lock you are holding. If you > are doing something else, please say what it is. > > Thanx, Paul > Paul & Peter, Thanks for looking into this stuff and providing me such valuable feedback, and RCU usage crashcourse. I believe rcu_dereference_protected() is what I want/need here, since this code is always called for pages which we hold locked (PG_locked bit). So, it brings me to ask you if the following usage looks sane enough to fix the well pointed issue, or if it's another misuse of RCU API: + mapping = rcu_dereference_protecetd(page->mapping, PageLocked(page)); + if (mapping) + mapping = mapping->assoc_mapping; _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
