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Date: Mon, 4 May 2020 08:05:03 -0400
From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
To: linux-kernel@...r.kernel.org
Cc: "Joel Fernandes (Google)" <joel@...lfernandes.org>,
Andy Lutomirski <luto@...nel.org>,
Frederic Weisbecker <fweisbec@...il.com>, frextrite@...il.com,
Ingo Molnar <mingo@...hat.com>,
Josh Triplett <josh@...htriplett.org>, kernel-team@...roid.com,
Lai Jiangshan <jiangshanlai@...il.com>,
madhuparnabhowmik04@...il.com,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
"Paul E. McKenney" <paulmck@...nel.org>, peterz@...radead.org,
Petr Mladek <pmladek@...e.com>, rcu@...r.kernel.org,
rostedt@...dmis.org, tglx@...utronix.de, vpillai@...italocean.com
Subject: [PATCH v3 3/5] rcu/tree: Clean up dynticks counter usage
The dynticks counter are confusing due to crowbar writes of
DYNTICK_IRQ_NONIDLE whose purpose is to detect half-interrupts (i.e. we
see rcu_irq_enter() but not rcu_irq_exit() due to a usermode upcall) and
if so then do a reset of the dyntick_nmi_nesting counters. This patch
tries to get rid of DYNTICK_IRQ_NONIDLE while still keeping the code
working, fully functional, and less confusing. The confusion recently
has even led to patches forgetting that DYNTICK_IRQ_NONIDLE was written
to which wasted lots of time.
The patch has the following changes:
(1) Use dynticks_nesting instead of dynticks_nmi_nesting for determining
outer most "EQS exit". This is needed to detect in
rcu_nmi_enter_common() if we have already EQS-exited, such as because of
a syscall. Currently we rely on a forced write of DYNTICK_IRQ_NONIDLE
from rcu_eqs_exit() for this purpose. This is one purpose of the
DYNTICK_IRQ_NONIDLE write (other than detecting half-interrupts).
However, we do not need to do that. dyntick_nesting already tells us that
we have EQS-exited so just use that thus removing the dependence of
dynticks_nmi_nesting for this purpose.
(2) Keep dynticks_nmi_nesting around because:
(a) rcu_is_cpu_rrupt_from_idle() needs to be able to detect first
interrupt nesting level.
(b) We need to detect half-interrupts till we are sure they're not an
issue. However, change the comparison to DYNTICK_IRQ_NONIDLE with 0.
(3) Since we got rid of DYNTICK_IRQ_NONIDLE, we also do cheaper
comparisons with zero instead for the code that keeps the tick on in
rcu_nmi_enter_common().
In the next patch, both of the concerns of (2) will be addressed and
then we can get rid of dynticks_nmi_nesting, however one step at a time.
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
kernel/rcu/rcu.h | 4 ----
kernel/rcu/tree.c | 58 +++++++++++++++++++++++++++--------------------
2 files changed, 34 insertions(+), 28 deletions(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index cf66a3ccd7573..b9f64abc48b85 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -12,10 +12,6 @@
#include <trace/events/rcu.h>
-/* Offset to allow distinguishing irq vs. task-based idle entry/exit. */
-#define DYNTICK_IRQ_NONIDLE ((LONG_MAX / 2) + 1)
-
-
/*
* Grace-period counter management.
*/
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index eb7a4d90b3b91..d6df8abdcc21f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -84,7 +84,6 @@
static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
.dynticks_nesting = 1,
- .dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
.dynticks = ATOMIC_INIT(1),
};
static struct rcu_state rcu_state = {
@@ -553,17 +552,19 @@ EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
/*
* Enter an RCU extended quiescent state, which can be either the
* idle loop or adaptive-tickless usermode execution.
- *
- * We crowbar the ->dynticks_nmi_nesting field to zero to allow for
- * the possibility of usermode upcalls having messed up our count
- * of interrupt nesting level during the prior busy period.
*/
static void rcu_eqs_enter(bool user)
{
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
- WARN_ON_ONCE(rdp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
- WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);
+ /*
+ * Entering usermode/idle from interrupt is not handled. These would
+ * mean usermode upcalls or idle exit happened from interrupts. Remove
+ * the warning by 2020.
+ */
+ if (WARN_ON_ONCE(rdp->dynticks_nmi_nesting != 0))
+ WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);
+
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
rdp->dynticks_nesting == 0);
if (rdp->dynticks_nesting != 1) {
@@ -641,26 +642,29 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
* (We are exiting an NMI handler, so RCU better be paying attention
* to us!)
*/
+ WARN_ON_ONCE(rdp->dynticks_nesting <= 0);
WARN_ON_ONCE(rdp->dynticks_nmi_nesting <= 0);
WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs());
+ WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
+ rdp->dynticks_nmi_nesting - 1);
/*
* If the nesting level is not 1, the CPU wasn't RCU-idle, so
* leave it in non-RCU-idle state.
*/
- if (rdp->dynticks_nmi_nesting != 1) {
+ if (rdp->dynticks_nesting != 1) {
trace_rcu_dyntick(TPS("StillNonIdle"), TPS("IRQ"),
- rdp->dynticks_nmi_nesting,
- rdp->dynticks_nmi_nesting - 2, atomic_read(&rdp->dynticks));
- WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
- rdp->dynticks_nmi_nesting - 2);
+ rdp->dynticks_nesting,
+ rdp->dynticks_nesting - 2, atomic_read(&rdp->dynticks));
+ WRITE_ONCE(rdp->dynticks_nesting, /* No store tearing. */
+ rdp->dynticks_nesting - 2);
return;
}
/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
- trace_rcu_dyntick(TPS("Start"), TPS("IRQ"), rdp->dynticks_nmi_nesting,
+ trace_rcu_dyntick(TPS("Start"), TPS("IRQ"), rdp->dynticks_nesting,
0, atomic_read(&rdp->dynticks));
- WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
+ WRITE_ONCE(rdp->dynticks_nesting, 0); /* Avoid store tearing. */
if (irq)
rcu_prepare_for_idle();
@@ -727,10 +731,6 @@ void rcu_irq_exit_irqson(void)
/*
* Exit an RCU extended quiescent state, which can be either the
* idle loop or adaptive-tickless usermode execution.
- *
- * We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to
- * allow for the possibility of usermode upcalls messing up our count of
- * interrupt nesting level during the busy period that is just now starting.
*/
static void rcu_eqs_exit(bool user)
{
@@ -755,8 +755,14 @@ static void rcu_eqs_exit(bool user)
rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
WRITE_ONCE(rdp->dynticks_nesting, 1);
- WARN_ON_ONCE(rdp->dynticks_nmi_nesting);
- WRITE_ONCE(rdp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
+
+ /*
+ * Exiting usermode/idle from interrupt is not handled. These would
+ * mean usermode upcalls or idle exit happened from interrupts. Remove
+ * the warning by 2020.
+ */
+ if (WARN_ON_ONCE(rdp->dynticks_nmi_nesting != 0))
+ WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);
}
/**
@@ -812,6 +818,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
/* Complain about underflow. */
+ WARN_ON_ONCE(rdp->dynticks_nesting < 0);
WARN_ON_ONCE(rdp->dynticks_nmi_nesting < 0);
/*
@@ -836,7 +843,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
incby = 1;
} else if (irq && tick_nohz_full_cpu(rdp->cpu) &&
- rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
+ !rdp->dynticks_nmi_nesting &&
READ_ONCE(rdp->rcu_urgent_qs) &&
!READ_ONCE(rdp->rcu_forced_tick)) {
// We get here only if we had already exited the extended
@@ -856,11 +863,14 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
}
trace_rcu_dyntick(incby == 1 ? TPS("End") : TPS("StillNonIdle"),
- TPS("IRQ"), rdp->dynticks_nmi_nesting,
- rdp->dynticks_nmi_nesting + incby, atomic_read(&rdp->dynticks));
+ TPS("IRQ"), rdp->dynticks_nesting,
+ rdp->dynticks_nesting + incby, atomic_read(&rdp->dynticks));
+
+ WRITE_ONCE(rdp->dynticks_nesting, /* Prevent store tearing. */
+ rdp->dynticks_nesting + incby);
WRITE_ONCE(rdp->dynticks_nmi_nesting, /* Prevent store tearing. */
- rdp->dynticks_nmi_nesting + incby);
+ rdp->dynticks_nmi_nesting + 1);
barrier();
}
--
2.26.2.526.g744177e7f7-goog
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