Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Apr 17, 2020 at 05:04:42PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-04-16 23:05:15 [-0400], Joel Fernandes wrote:
> > On Thu, Apr 16, 2020 at 11:34:44PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2020-04-16 14:00:57 [-0700], Paul E. McKenney wrote:
> > > > 
> > > > We might need different calling-context restrictions for the two variants
> > > > of kfree_rcu().  And we might need to come up with some sort of lockdep
> > > > check for "safe to use normal spinlock in -rt".
> > > 
> > > Oh. We do have this already, it is called CONFIG_PROVE_RAW_LOCK_NESTING.
> > > This one will scream if you do
> > > 	raw_spin_lock();
> > > 	spin_lock();
> > > 
> > > Sadly, as of today, there is code triggering this which needs to be
> > > addressed first (but it is one list of things to do).
> > > 
> > > Given the thread so far, is it okay if I repost the series with
> > > migrate_disable() instead of accepting a possible migration before
> > > grabbing the lock? I would prefer to avoid the extra RT case (avoiding
> > > memory allocations in a possible atomic context) until we get there.
> > 
> > I prefer something like the following to make it possible to invoke
> > kfree_rcu() from atomic context considering call_rcu() is already callable
> > from such contexts. Thoughts?
> 
> So it looks like it would work. However, could we please delay this
> until we have an actual case on RT? I just added
> 	WARN_ON(!preemptible());

I am not sure if waiting for it to break in the future is a good idea. I'd
rather design it in a forward thinking way. There could be folks replacing
"call_rcu() + kfree in a callback" with kfree_rcu() for example. If they were
in !preemptible(), we'd break on page allocation.

Also as a sidenote, the additional pre-allocation of pages that Vlad is
planning on adding would further reduce the need for pages from the page
allocator.

Paul, what is your opinion on this?

thanks,

 - Joel


> 
> to kfree_call_rcu() on v5.6.4-rt4 and nothing triggered.
> 
> This is the list of users I had (just to figure out if this is used at
> all):
> - addrconf_ifdown
> - cgroup_free
> - cgroup_migrate_finish
> - css_task_iter_end
> - disk_expand_part_tbl
> - drop_sysctl_table
> - __hw_addr_flush
> - inetdev_event
> - ip6addrlbl_net_exit
> - ip6addrlbl_net_exit
> - ops_exit_list.isra.0
> - rtnl_register_internal
> - simple_set_acl
> - swevent_hlist_put_cpu
> - timerfd_release
> - vfs_rename
> - __xfs_set_acl
> 
> Sebastian