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 45e58f14b1ce..323954363389 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. */ -- 2.31.1.189.g2e36527f23
