[tip: core/rcu] rcu: Remove obsolete rcu_read_unlock() deadlock commentary

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     0223846010750e28e4330f1beefb5564ba406ef7
Gitweb:        https://git.kernel.org/tip/0223846010750e28e4330f1beefb5564ba406ef7
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Thu, 29 Apr 2021 11:30:49 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 13 May 2021 09:13:23 -07:00

rcu: Remove obsolete rcu_read_unlock() deadlock commentary

The deferred quiescent states resulting from the consolidation of RCU-bh
and RCU-sched into RCU means that rcu_read_unlock() will no longer attempt
to acquire scheduler locks if interrupts were disabled across that call
to rcu_read_unlock().  The cautions in the rcu_read_unlock() header
comment are therefore obsolete.  This commit therefore removes them.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 include/linux/rcupdate.h | 33 ++++++---------------------------
 1 file changed, 6 insertions(+), 27 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f0eecb9..d9680b7 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -702,33 +702,12 @@ static __always_inline void rcu_read_lock(void)
 /**
  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
  *
- * In most situations, rcu_read_unlock() is immune from deadlock.
- * However, in kernels built with CONFIG_RCU_BOOST, rcu_read_unlock()
- * is responsible for deboosting, which it does via rt_mutex_unlock().
- * Unfortunately, this function acquires the scheduler's runqueue and
- * priority-inheritance spinlocks.  This means that deadlock could result
- * if the caller of rcu_read_unlock() already holds one of these locks or
- * any lock that is ever acquired while holding them.
- *
- * That said, RCU readers are never priority boosted unless they were
- * preempted.  Therefore, one way to avoid deadlock is to make sure
- * that preemption never happens within any RCU read-side critical
- * section whose outermost rcu_read_unlock() is called with one of
- * rt_mutex_unlock()'s locks held.  Such preemption can be avoided in
- * a number of ways, for example, by invoking preempt_disable() before
- * critical section's outermost rcu_read_lock().
- *
- * Given that the set of locks acquired by rt_mutex_unlock() might change
- * at any time, a somewhat more future-proofed approach is to make sure
- * that that preemption never happens within any RCU read-side critical
- * section whose outermost rcu_read_unlock() is called with irqs disabled.
- * This approach relies on the fact that rt_mutex_unlock() currently only
- * acquires irq-disabled locks.
- *
- * The second of these two approaches is best in most situations,
- * however, the first approach can also be useful, at least to those
- * developers willing to keep abreast of the set of locks acquired by
- * rt_mutex_unlock().
+ * In almost all situations, rcu_read_unlock() is immune from deadlock.
+ * In recent kernels that have consolidated synchronize_sched() and
+ * synchronize_rcu_bh() into synchronize_rcu(), this deadlock immunity
+ * also extends to the scheduler's runqueue and priority-inheritance
+ * spinlocks, courtesy of the quiescent-state deferral that is carried
+ * out when rcu_read_unlock() is invoked with interrupts disabled.
  *
  * See rcu_read_lock() for more information.
  */