Messages in this thread | | | Date | Mon, 17 May 2010 16:05:33 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 23/23] vhost: add __rcu annotations |
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On Mon, May 17, 2010 at 06:00:25PM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Mon, May 17, 2010 at 11:33:49PM +0300, Michael S. Tsirkin wrote: > > > On Thu, May 13, 2010 at 08:23:40AM -0700, Paul E. McKenney wrote: > > > > On Thu, May 13, 2010 at 03:07:23PM +0200, Peter Zijlstra wrote: > > > > > On Wed, 2010-05-12 at 16:00 -0700, Paul E. McKenney wrote: > > > > > > Any thoughts? One approach would be to create a separate lockdep class > > > > > > for vhost workqueue state, similar to the approach used in instrument > > > > > > rcu_read_lock() and friends. > > > > > > > > > > workqueue_struct::lockdep_map, its held while executing worklets. > > > > > > > > > > lock_is_held(&vhost_workqueue->lockdep_map), should do as you want. > > > > > > > > Thank you, Peter!!! > > > > > > > > Thanx, Paul > > > > > > vhost in fact does flush_work rather than > > > flush_workqueue, so while for now everything runs > > > from vhost_workqueue in theory nothing would break > > > if we use some other workqueue or even a combination > > > thereof. > > > > > > I guess when/if this happens, we could start by converting > > > to _raw and then devise a solution. > > > > If there are a small finite number of work queues involved, we can > > easily do something like: > > > > #ifdef CONFIG_PROVE_RCU > > int in_vhost_workqueue(void) > > { > > return in_workqueue_context(vhost_workqueue) || > > in_workqueue_context(vhost_other_workqueue) || > > in_workqueue_context(yet_another_vhost_workqueue); > > } > > #endif /* #ifdef CONFIG_PROVE_RCU */ > > > > Seem reasonable? > > > > > By the way what would be really nice is if we had a way > > > to trap when rcu protected pointer is freed without a flush > > > while some reader is running. Current annotation does not > > > allow this, does it? > > > > Right now, it does not, but I wonder if something like Thomas's and > > Mathieu's debugobjects work could be brought to bear on this problem? > > This would need to be implemented in vhost, as synchronize_rcu() has > > no way to know what memory it is flushing, nor does flush_work(). > > We can think of my recent debugobjects addition as a small state machine > that is described by the code that owns the objects. At each state > transition, the code passes the expected state as well as the next > state. > > The current implementation can only keep track of a single "state" per > object at once. This should be extended to be able to count the number > RCU read side C.S. in flight that are accessing to an object.
Not a problem, as vhost doesn't use call_rcu(). So there won't be a conflict between different debugobjects views of the same memory.
> We could use a hook in rcu_dereference (which knows about the object) > and a hook in rcu_read_unlock (which determines the end of valid object > use). > > We should hook into rcu_assign_pointer() to detect RCU structure > privatization. It should put these objects in a "privatized" hash table. > > We should also hook into synchronize_rcu/sched() to remove the > privatized structures from the privatized hash. > > A hook in "kfree" (maybe a new rcu_free(void (fctptr*)(void *)) wrapper ?) > would call a debugobject hook that would lookup the "privatized" hash. > If it contains the object to free, we check if there are RCU read-side > C.S. in flight using this object at the same time, and show an error if > both are true.
I believe that we can't bury this into the RCU primitives, because rcu_read_unlock() doesn't know what objects were referenced in the RCU read-side critical section.
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?
Thanx, Paul
> Thoughts ? > > Mathieu > > > > > > Thanx, Paul > > -- > Mathieu Desnoyers > Operating System Efficiency R&D Consultant > EfficiOS Inc. > http://www.efficios.com
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