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
