Revert to fix RCU-related deadlock/softlockup

Hello, Ingo,

This series has a single revert from the ill-starred attempt to inline __rcu_read_lock() for preemptible RCU. Without this revert, on mainline kernels using CONFIG_RCU_BOOST there is a low-probability deadlock on the runqueue locks, but one that actually appeared in Sasha Levin's testing. With the revert, and with an diagnostic patch that increased probability of the deadlock to a MTBF of roughly 10 seconds, Sasha's tests ran for two days with no failure.

The sequence of events leading to the deadlock is as follows:

1. A task enters an RCU read-side critical section, and is both
preempted and subjected to RCU priority boosting.
2. The task starts to exit its RCU read-side critical section,
but is preempted in __rcu_read_unlock() just after the assignment setting t->rcu_read_lock_nesting to INT_MIN. (The diagnostic patch mentioned above expands this window by ten microseconds, and is available in -rcu as a debug option queued for 3.7.)
3. The task enters the scheduler, where it acquires the corresponding
runqueue lock, then invokes rcu_switch_from() which in turn invokes rcu_preempt_note_context_switch(), which in turn invokes rcu_read_unlock_special(), which attempts to deboost the task.
4. The attempt to deboost the task recursively enters the scheduler
with a runqueue lock held, which can result in deadlock.

The revert moves the point at which rcu_preempt_note_context_switch() is called to a point in the scheduler code before the runqueue lock is acquired, avoiding the deadlock.

This pull is marked "RFC" because CONFIG_RCU_BOOST=y is not used much outside of the real-time community. I will be sending another pull request later today (Pacific Time) for 3.6 RCU commits, which will include this commit as well. Your choice. ;-)

This change is available in the git repository at:
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/urgent

Thanx, Paul

------------------>
Paul E. McKenney (1):
Revert "rcu: Move PREEMPT_RCU preemption to switch_to() invocation"

 arch/um/drivers/mconsole_kern.c |    1 -
 include/linux/rcupdate.h        |    1 -
 include/linux/rcutiny.h         |    6 ++++++
 include/linux/sched.h           |   10 ----------
 kernel/rcutree.c                |    1 +
 kernel/rcutree.h                |    1 +
 kernel/rcutree_plugin.h         |   14 +++++++++++---
 kernel/sched/core.c             |    1 -
 8 files changed, 19 insertions(+), 16 deletions(-)

It got introduced in this cycle so I agree with you that the fix for the regression is justified.

Pulled, thanks Paul!

Ingo