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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
