> 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:
> > > > * 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.
> >
> > Not quite sure I follow you here.
>
> vhost uses only synchronize_rcu() and flush_work(). The existing
> debugobjects tagging would therefore be unaware of the actual object,
> instead tagging the rcu_head that synchronize_rcu() allocated on
> the stack, and being out of the picture completely in the case of
> flush_work().
>
> Either way, RCU is completely unaware of exactly which structure is
> being pushed through a grace period, so RCU's debugobjects tagging cannot
> possibly conflict with any tagging that vhost does.
