On Sun, Aug 18, 2019 at 07:38:39PM -0400, Joel Fernandes wrote: > On Sun, Aug 18, 2019 at 04:31:35PM -0700, Paul E. McKenney wrote: > > On Sun, Aug 18, 2019 at 06:35:11PM -0400, Joel Fernandes wrote: > > > On Sun, Aug 18, 2019 at 06:32:30PM -0400, Joel Fernandes wrote: > > > > On Sun, Aug 18, 2019 at 03:12:10PM -0700, Paul E. McKenney wrote: > > > > > On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote: > > > > > > When we're in hard interrupt context in rcu_read_unlock_special(), we > > > > > > can still benefit from invoke_rcu_core() doing wake ups of rcuc > > > > > > threads when the !use_softirq parameter is passed. This is safe > > > > > > to do so because: > > > > > > > > > > > > 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit > > > > > > introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe > > > > > > conditions in rcu_read_unlock_special()") by checking for the same in > > > > > > this patch. > > > > > > > > > > > > 2. in_irq() implies in_interrupt() which implies raising softirq will > > > > > > not do any wake ups. > > > > > > > > > > > > The rcuc thread which is awakened will run when the interrupt returns. > > > > > > > > > > > > We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups > > > > > > if expedited") thus doing the rcuc awakening only when none of the > > > > > > following are true: > > > > > > 1. Critical section is blocking an expedited GP. > > > > > > 2. A nohz_full CPU. > > > > > > If neither of these cases are true (exp == false), then the "else" block > > > > > > will run to do the irq_work stuff. > > > > > > > > > > > > This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify > > > > > > rcu_read_unlock_special() deferred wakeups") with an additional in_irq() > > > > > > check added. > > > > > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > > > > > > > > > OK, I will bite... If it is safe to wake up an rcuc kthread, why > > > > > is it not safe to do raise_softirq()? > > > > > > > > Because raise_softirq should not be done and/or doesn't do anything > > > > if use_softirq == false. In fact, RCU_SOFTIRQ doesn't even existing if > > > > use_softirq == false. The "else if" condition of this patch uses for > > > > use_softirq. > > > > > > > > Or, did I miss your point? > > > > I am concerned that added "else if" condition might not be sufficient > > to eliminate all possible cases of the caller holding a scheduler lock, > > which could result in deadlock in the ensuing wakeup. Might be me missing > > something, but such deadlocks have been a recurring problem in the past. > > I thought that was the whole point of the > rcu_read_unlock_special.b.deferred_qs bit that was introduced in > 23634ebc1d94. We are checking that bit in the "else if" here as well. So this > should be no less immune to scheduler deadlocks any more than the preceding > "else if" where we are checking this bit. I would have much more confidence in a line of reasoning that led to "immune to scheduler deadlocks" than one that led to "no less immune to scheduler deadlocks". ;-) > > Also, your commit log's point #2 is "in_irq() implies in_interrupt() > > which implies raising softirq will not do any wake ups." This mention > > of softirq seems a bit odd, given that we are going to wake up a rcuc > > kthread. Of course, this did nothing to quell my suspicions. ;-) > > Yes, I should delete this #2 from the changelog since it is not very relevant > (I feel now). My point with #2 was that even if were to raise a softirq > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this > path anyway since in_irq() implies in_interrupt(). Please! Could you also add a first-principles explanation of why the added condition is immune from scheduler deadlocks? Thanx, Paul