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rcu: Remove obsolete rcu_read_unlock() deadlock commentary

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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@kernel.org>
This commit is contained in:
Paul E. McKenney 2021-04-29 11:30:49 -07:00
parent 1893afd634
commit 0223846010
1 changed files with 6 additions and 27 deletions
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33
include/linux/rcupdate.h
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@ -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.
*/