Re: [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting
From: Lai Jiangshan
Date: Sun Nov 17 2019 - 20:54:39 EST
On 2019/11/18 5:53 äå, Paul E. McKenney wrote:
On Sat, Nov 16, 2019 at 09:04:56PM +0800, Lai Jiangshan wrote:
On 2019/11/1 8:33 äå, Paul E. McKenney wrote:
On Thu, Oct 31, 2019 at 10:08:03AM +0000, Lai Jiangshan wrote:
Negative ->rcu_read_lock_nesting was introduced to prevent
scheduler deadlock which was just prevented by deferred qs.
So negative ->rcu_read_lock_nesting is useless now and
rcu_read_unlock() can be simplified.
And negative ->rcu_read_lock_nesting is bug-prone,
it is good to kill it.
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree_exp.h | 30 ++----------------------------
kernel/rcu/tree_plugin.h | 21 +++++----------------
2 files changed, 7 insertions(+), 44 deletions(-)
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index c0d06bce35ea..9dcbd2734620 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -621,11 +621,11 @@ static void rcu_exp_handler(void *unused)
* report the quiescent state, otherwise defer.
*/
if (!t->rcu_read_lock_nesting) {
+ rdp->exp_deferred_qs = true;
if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
rcu_dynticks_curr_cpu_in_eqs()) {
- rcu_report_exp_rdp(rdp);
+ rcu_preempt_deferred_qs(t);
} else {
- rdp->exp_deferred_qs = true;
set_tsk_need_resched(t);
set_preempt_need_resched();
}
@@ -646,32 +646,6 @@ static void rcu_exp_handler(void *unused)
WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
return;
}
-
- /*
- * The final and least likely case is where the interrupted
- * code was just about to or just finished exiting the RCU-preempt
- * read-side critical section, and no, we can't tell which.
- * So either way, set ->deferred_qs to flag later code that
- * a quiescent state is required.
- *
- * If the CPU is fully enabled (or if some buggy RCU-preempt
- * read-side critical section is being used from idle), just
- * invoke rcu_preempt_deferred_qs() to immediately report the
- * quiescent state. We cannot use rcu_read_unlock_special()
- * because we are in an interrupt handler, which will cause that
- * function to take an early exit without doing anything.
- *
- * Otherwise, force a context switch after the CPU enables everything.
- */
- rdp->exp_deferred_qs = true;
- if (rcu_preempt_need_deferred_qs(t) &&
- (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
- WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()))) {
- rcu_preempt_deferred_qs(t);
- } else {
- set_tsk_need_resched(t);
- set_preempt_need_resched();
- }
}
/* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dbded2b8c792..c62631c79463 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -344,8 +344,6 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
}
/* Bias and limit values for ->rcu_read_lock_nesting. */
-#define RCU_NEST_BIAS INT_MAX
-#define RCU_NEST_NMAX (-INT_MAX / 2)
#define RCU_NEST_PMAX (INT_MAX / 2)
/*
@@ -373,21 +371,15 @@ void __rcu_read_unlock(void)
{
struct task_struct *t = current;
- if (t->rcu_read_lock_nesting != 1) {
- --t->rcu_read_lock_nesting;
- } else {
+ if (--t->rcu_read_lock_nesting == 0) {
barrier(); /* critical section before exit code. */
- t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
- barrier(); /* assign before ->rcu_read_unlock_special load */
But if we take an interrupt here, and the interrupt handler contains
an RCU read-side critical section, don't we end up in the same hole
that resulted in this article when the corresponding rcu_read_unlock()
executes? https://lwn.net/Articles/453002/
Hello, Paul
I'm replying the email of V1, which is relying on deferred_qs changes
in [PATCH 07/11] (V1).
([PATCH 04/11](V1) relies on it too as you pointed out)
I hope I can answer the question wrt https://lwn.net/Articles/453002/
maybe partially.
With the help of deferred_qs mechanism and the special.b.deferred_qs
bit, I HOPED rcu_read_unlock_special() can find if itself is
risking in scheduler locks via special.b.deferred_qs bit.
--t->rcu_read_lock_nesting;
//outmost rcu c.s, rcu_read_lock_nesting is 0. but special is not zero
INTERRUPT
// the fallowing code will normally be in_interrupt()
// or NOT in_interrupt() when wakeup_softirqd() in invoke_softirq()
// or NOT in_interrupt() when preempt_shedule_irq()
// or other cases I missed.
scheduler_lock()
rcu_read_lock()
rcu_read_unlock()
// special has been set but with no special.b.deferred_qs
rcu_read_unlock_special()
raise_softirq_irqoff()
wake_up() when !in_interrupt() // dead lock
preempt_shedule_irq() is guaranteed to clear rcu_read_unlock_special
when rcu_read_lock_nesting = 0 before calling into scheduler locks.
But, at least, what caused my hope to be failed was the case
wakeup_softirqd() in invoke_softirq() (which was once protected by
softirq in about 2 years between ec433f0c5152 and facd8b80c67a).
I don't think it is hard to fix it if we keep using
special.b.deferred_qs as this V1 series.
It is quite possible that special.b.deferred_qs might be useful
for debugging. But it should now be possible to take care of the
nohz_full issue for expedited grace periods, which might in turn allow
rcu_read_unlock_special() to avoid acquiring scheduler locks.
This could avoid the need for negative ->rcu_read_lock_nesting,
in turn allowing your simplified _rcu_read_unlock().
Would you like to do the expedited grace-period modifications, or
would you rather that I do so?
Hello, Paul
To be honest, I didn't known there was special issue about
nohz_full with expedited grace periods until several days before
you told me. I just thought that it is requested to be expedited
so that we need to wake up something to handle it ASAP.
IOW, I'm not in a position to do the expedited grace-period
modifications before I learnt enough about it. I would be very
obliged that you do so. I believe it will be a better solution
than this one or the one in V2 relying on preempt_count.
Thanks
Lai