LinuxKPI: Do not wait for a grace period in rcu_barrier()

[wulf]

Summary

Linux docs explicitly state that this is not required [1]:

"Important note: The rcu_barrier() function is not, repeat, not,
obligated to wait for a grace period. It is instead only required to
wait for RCU callbacks that have already been posted. Therefore, if
there are no RCU callbacks posted anywhere in the system, rcu_barrier()
is within its rights to return immediately. Even if there are
callbacks posted, rcu_barrier() does not necessarily need to wait for
a grace period."

P.S. This makes my current workaround[2] for i915 double-free related
crashes on drm-kmod v5.5 cheaper.

[1] https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html
[2] https://github.com/wulf7/drm-kmod/commit/cf961531da0c6f58f2ba0aaabf4ce423be820fd4

[emaste]

sys/compat/linuxkpi/common/src/linux_rcu.c

369–375

What do you think about noting (with a comment) that linux_synchronize_rcu(type) is not necessary and explicitly omitted? It's the sort of thing that one might expect is needed (as evidenced by the existing code) so seems like it might make sense to mention.

[• hselasky]

Change looks good.

[wulf]

sys/compat/linuxkpi/common/src/linux_rcu.c

369–375

What do you think about noting (with a comment) that linux_synchronize_rcu(type) is not necessary and explicitly omitted? It's the sort of thing that one might expect is needed (as evidenced by the existing code) so seems like it might make sense to mention.

I'll add such a comment