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Subject[GIT RFC PULL rcu/urgent] 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:// 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(-)

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