Messages in this thread |  | | | Date | Tue, 18 May 2010 08:07:41 -0700 | | From | "Paul E. McKenney" <> | | Subject | Re: [PATCH RFC tip/core/rcu 23/23] vhost: add __rcu annotations |
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On Tue, May 18, 2010 at 05:25:57PM +0300, Michael S. Tsirkin wrote: > On Tue, May 18, 2010 at 07:20:08AM -0700, Paul E. McKenney wrote: > > On Mon, May 17, 2010 at 09:35:28PM -0400, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > On Mon, May 17, 2010 at 07:40:25PM -0400, Mathieu Desnoyers wrote: > > > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > > > On Mon, May 17, 2010 at 06:00:25PM -0400, Mathieu Desnoyers wrote: > > > > [ . . . ] > > > > > > > > But perhaps we should be simply treating this as a use-after-free > > > > > > problem, so that RCU is not directly involved. Isn't that the standard > > > > > > use of debugobjects anyway? > > > > > > > > > > OK so we could tie "rcu_dereference" do debugobjects, and free would be > > > > > a standard free. Yes, I think it could be done. It looks a bit like the > > > > > memory allocation debugging code. If we know that a certain > > > > > rcu_dereference always access dynamically allocated memory, we could > > > > > probably add some checks there based on the memory allocator debug > > > > > objects. > > > > > > > > We probably need vhost to add code at the end of the relevant RCU > > > > read-side critical section checking that the pointers returned by > > > > any rcu_dereference() calls still point to valid memory. Don't get > > > > me wrong, your approach could find bugs in which someone forgot to > > > > remove the RCU-protected structure from a public list, but it could > > > > not detect failure to wait a grace period between the time of removal > > > > and the time of freeing. > > > > > > Good point too. So something like a new rcu_unreference() (or feel free > > > to find any better name) ;) that would be compiled out normally, but > > > would call into debugobjects might do the trick. We would have to add > > > these annotations to match every rcu_dereference() though, might means a > > > lot of new lines of code. On the plus side, that looks like a good audit > > > of RCU read-side use. ;) > > > > My first thought is that we have added quite a bit of RCU consistency > > check code in the past few months, so we should see what bugs they find > > and what bugs escape. It is all too easy to create consistency check > > code that is more trouble than it is worth. > > Right. Do the patches that started this discussion catch anything BTW?
All three approaches have found some bugs.
> > But in the meantime, let's see what would be required to check for > > failures to insert grace-period delays: > > > > o There would need to be something like rcu_unreference(), > > rcu_no_more_readers() or some such after the grace period. > > The update side would then become something like the following: > > > > oldp = rcu_dereference_protected(gp, &mylock); > > rcu_assign_pointer(gp, newp); > > synchronize_rcu(); > > rcu_no_more_readers(oldp); > > kfree(oldp); > > > > o There would need to be something to check all of the pointers > > traversed in the read-side critical sections: > > > > rcu_read_lock(); > > ... > > p1 = rcu_dereference(gp1->field1); > > ... > > p2 = rcu_dereference(gp2->field2); > > ... > > > > rcu_validate(p1); > > rcu_validate(p2); > > rcu_read_unlock(); > > > > what does rcu_validate do?
It checks to make sure that the pointer still points to something valid.
> > One thing that bothers me about this is that we are forcing the developer > > to do a lot of extra typing. For example, rcu_no_more_readers() is in > > a truth-and-beauty sense redundant with kfree() -- why type both? > > With kfree, yes. We could stick rcu_no_more_readers in kfree I guess?
But why not just use the existing debugobjects? You can just use something like this:
debug_check_no_obj_freed(p1, sizeof(*p1));
in place of:
rcu_validate(p1);
Of course, if you are using your own custom allocator, you will need to put the allocation/free checks in, same as slab and the others currently do.
Thanx, Paul
> > The > > same could be said about rcu_validate() and rcu_read_unlock(), but nested > > RCU read-side critical sections make this difficult. > > Or am I misunderstanding what you are suggesting? > > > > Thanx, Paul
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