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Message-ID: <20200215134208.GA9879@paulmck-ThinkPad-P72>
Date: Sat, 15 Feb 2020 05:42:08 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...icios.com, josh@...htriplett.org,
tglx@...utronix.de, peterz@...radead.org, dhowells@...hat.com,
edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
joel@...lfernandes.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs
before GP kthread is running
On Sat, Feb 15, 2020 at 03:01:11AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 14, 2020 at 10:53:05PM -0500, Steven Rostedt wrote:
> > On Fri, 14 Feb 2020 15:55:59 -0800
> > paulmck@...nel.org wrote:
> >
> > > @@ -1252,10 +1252,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
> > > */
> > > static void rcu_gp_kthread_wake(void)
> > > {
> > > - if ((current == rcu_state.gp_kthread &&
> > > + if ((current == READ_ONCE(rcu_state.gp_kthread) &&
> > > !in_irq() && !in_serving_softirq()) ||
> > > !READ_ONCE(rcu_state.gp_flags) ||
> > > - !rcu_state.gp_kthread)
> > > + !READ_ONCE(rcu_state.gp_kthread))
> > > return;
> >
> > This looks buggy. You have two instances of
> > READ_ONCE(rcu_state.gp_thread), which means they can be different. Is
> > that intentional?
>
> It might well be a bug, but let's see...
>
> The rcu_state.gp_kthread field is initially NULL and transitions only once
> to the non-NULL pointer to the RCU grace-period kthread's task_struct
> structure. So yes, this does work, courtesy of the compiler not being
> allowed to change the order of READ_ONCE() instances and conherence-order
> rules for READ_ONCE() and WRITE_ONCE().
>
> But it would clearly be way better to do just one READ_ONCE() into a
> local variable and test that local variable twice.
>
> I will make this change, and thank you for calling my attention to it!
And does the following V2 look better?
Thanx, Paul
------------------------------------------------------------------------
commit 35f7c539d30d5b595718302d07334146f8eb7304
Author: Paul E. McKenney <paulmck@...nel.org>
Date: Tue Jan 21 12:30:22 2020 -0800
rcu: Don't flag non-starting GPs before GP kthread is running
Currently rcu_check_gp_start_stall() complains if a grace period takes
too long to start, where "too long" is roughly one RCU CPU stall-warning
interval. This has worked well, but there are some debugging Kconfig
options (such as CONFIG_EFI_PGT_DUMP=y) that can make booting take a
very long time, so much so that the stall-warning interval has expired
before RCU's grace-period kthread has even been spawned.
This commit therefore resets the rcu_state.gp_req_activity and
rcu_state.gp_activity timestamps just before the grace-period kthread
is spawned, and modifies the checks and adds ordering to ensure that
if rcu_check_gp_start_stall() sees that the grace-period kthread
has been spawned, that it will also see the resets applied to the
rcu_state.gp_req_activity and rcu_state.gp_activity timestamps.
Reported-by: Qian Cai <cai@....pw>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
[ paulmck: Fix whitespace issues reported by Qian Cai. ]
Tested-by: Qian Cai <cai@....pw>
[ paulmck: Simplify grace-period wakeup check per Steve Rostedt feedback. ]
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 62383ce..4a4a975 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1202,7 +1202,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT);
WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
- if (!rcu_state.gp_kthread) {
+ if (!READ_ONCE(rcu_state.gp_kthread)) {
trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
goto unlock_out;
}
@@ -1237,12 +1237,13 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
}
/*
- * Awaken the grace-period kthread. Don't do a self-awaken (unless in
- * an interrupt or softirq handler), and don't bother awakening when there
- * is nothing for the grace-period kthread to do (as in several CPUs raced
- * to awaken, and we lost), and finally don't try to awaken a kthread that
- * has not yet been created. If all those checks are passed, track some
- * debug information and awaken.
+ * Awaken the grace-period kthread. Don't do a self-awaken (unless in an
+ * interrupt or softirq handler, in which case we just might immediately
+ * sleep upon return, resulting in a grace-period hang), and don't bother
+ * awakening when there is nothing for the grace-period kthread to do
+ * (as in several CPUs raced to awaken, we lost), and finally don't try
+ * to awaken a kthread that has not yet been created. If all those checks
+ * are passed, track some debug information and awaken.
*
* So why do the self-wakeup when in an interrupt or softirq handler
* in the grace-period kthread's context? Because the kthread might have
@@ -1252,10 +1253,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
*/
static void rcu_gp_kthread_wake(void)
{
- if ((current == rcu_state.gp_kthread &&
- !in_irq() && !in_serving_softirq()) ||
- !READ_ONCE(rcu_state.gp_flags) ||
- !rcu_state.gp_kthread)
+ struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
+
+ if ((current == t && !in_irq() && !in_serving_softirq()) ||
+ !READ_ONCE(rcu_state.gp_flags) || !t)
return;
WRITE_ONCE(rcu_state.gp_wake_time, jiffies);
WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));
@@ -3554,7 +3555,10 @@ static int __init rcu_spawn_gp_kthread(void)
}
rnp = rcu_get_root();
raw_spin_lock_irqsave_rcu_node(rnp, flags);
- rcu_state.gp_kthread = t;
+ WRITE_ONCE(rcu_state.gp_activity, jiffies);
+ WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
+ // Reset .gp_activity and .gp_req_activity before setting .gp_kthread.
+ smp_store_release(&rcu_state.gp_kthread, t); /* ^^^ */
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
wake_up_process(t);
rcu_spawn_nocb_kthreads();
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 488b71d..16ad7ad 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -578,6 +578,7 @@ void show_rcu_gp_kthreads(void)
unsigned long jw;
struct rcu_data *rdp;
struct rcu_node *rnp;
+ struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
j = jiffies;
ja = j - READ_ONCE(rcu_state.gp_activity);
@@ -585,8 +586,7 @@ void show_rcu_gp_kthreads(void)
jw = j - READ_ONCE(rcu_state.gp_wake_time);
pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
rcu_state.name, gp_state_getname(rcu_state.gp_state),
- rcu_state.gp_state,
- rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,
+ rcu_state.gp_state, t ? t->state : 0x1ffffL,
ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),
(long)READ_ONCE(rcu_state.gp_seq),
(long)READ_ONCE(rcu_get_root()->gp_seq_needed),
@@ -633,7 +633,8 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
- READ_ONCE(rnp_root->gp_seq_needed)))
+ READ_ONCE(rnp_root->gp_seq_needed)) ||
+ !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
return;
j = jiffies; /* Expensive access, and in common case don't get here. */
if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
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