* [PATCH 1/2] cgroup: Use irqsave in cgroup_rstat_flush_locked().
@ 2022-03-01 12:21 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 10+ messages in thread
From: Sebastian Andrzej Siewior @ 2022-03-01 12:21 UTC (permalink / raw)
To: cgroups, linux-mm
Cc: Andrew Morton, Johannes Weiner, Tejun Heo, Zefan Li,
Thomas Gleixner, Sebastian Andrzej Siewior
All callers of cgroup_rstat_flush_locked() acquire cgroup_rstat_lock
either with spin_lock_irq() or spin_lock_irqsave().
cgroup_rstat_flush_locked() itself acquires cgroup_rstat_cpu_lock which
is a raw_spin_lock. This lock is also acquired in cgroup_rstat_updated()
in IRQ context and therefore requires _irqsave() locking suffix in
cgroup_rstat_flush_locked().
Since there is no difference between spin_lock_t and raw_spin_lock_t
on !RT lockdep does not complain here. On RT lockdep complains because
the interrupts were not disabled here and a deadlock is possible.
Acquire the raw_spin_lock_t with disabled interrupts.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/cgroup/rstat.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 9d331ba44870a..53b771c20ee50 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -153,8 +153,9 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock,
cpu);
struct cgroup *pos = NULL;
+ unsigned long flags;
- raw_spin_lock(cpu_lock);
+ raw_spin_lock_irqsave(cpu_lock, flags);
while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
struct cgroup_subsys_state *css;
@@ -166,7 +167,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
css->ss->css_rstat_flush(css, cpu);
rcu_read_unlock();
}
- raw_spin_unlock(cpu_lock);
+ raw_spin_unlock_irqrestore(cpu_lock, flags);
/* if @may_sleep, play nice and yield if necessary */
if (may_sleep && (need_resched() ||
--
2.35.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH 1/2] cgroup: Use irqsave in cgroup_rstat_flush_locked().
@ 2022-03-01 12:21 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 10+ messages in thread
From: Sebastian Andrzej Siewior @ 2022-03-01 12:21 UTC (permalink / raw)
To: cgroups-u79uwXL29TY76Z2rM5mHXA, linux-mm-Bw31MaZKKs3YtjvyW6yDsg
Cc: Andrew Morton, Johannes Weiner, Tejun Heo, Zefan Li,
Thomas Gleixner, Sebastian Andrzej Siewior
All callers of cgroup_rstat_flush_locked() acquire cgroup_rstat_lock
either with spin_lock_irq() or spin_lock_irqsave().
cgroup_rstat_flush_locked() itself acquires cgroup_rstat_cpu_lock which
is a raw_spin_lock. This lock is also acquired in cgroup_rstat_updated()
in IRQ context and therefore requires _irqsave() locking suffix in
cgroup_rstat_flush_locked().
Since there is no difference between spin_lock_t and raw_spin_lock_t
on !RT lockdep does not complain here. On RT lockdep complains because
the interrupts were not disabled here and a deadlock is possible.
Acquire the raw_spin_lock_t with disabled interrupts.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
kernel/cgroup/rstat.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 9d331ba44870a..53b771c20ee50 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -153,8 +153,9 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock,
cpu);
struct cgroup *pos = NULL;
+ unsigned long flags;
- raw_spin_lock(cpu_lock);
+ raw_spin_lock_irqsave(cpu_lock, flags);
while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
struct cgroup_subsys_state *css;
@@ -166,7 +167,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
css->ss->css_rstat_flush(css, cpu);
rcu_read_unlock();
}
- raw_spin_unlock(cpu_lock);
+ raw_spin_unlock_irqrestore(cpu_lock, flags);
/* if @may_sleep, play nice and yield if necessary */
if (may_sleep && (need_resched() ||
--
2.35.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH 2/2] mm: workingset: Replace IRQ-off check with a lockdep assert.
@ 2022-03-01 12:21 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 10+ messages in thread
From: Sebastian Andrzej Siewior @ 2022-03-01 12:21 UTC (permalink / raw)
To: cgroups, linux-mm
Cc: Andrew Morton, Johannes Weiner, Tejun Heo, Zefan Li,
Thomas Gleixner, Sebastian Andrzej Siewior
Commit
68d48e6a2df57 ("mm: workingset: add vmstat counter for shadow nodes")
introduced an IRQ-off check to ensure that a lock is held which also
disabled interrupts. This does not work the same way on PREEMPT_RT
because none of the locks, that are held, disable interrupts.
Replace this check with a lockdep assert which ensures that the lock is
held.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
mm/workingset.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/workingset.c b/mm/workingset.c
index 2e4fd7c3296fe..8a3828acc0bfd 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -434,6 +434,8 @@ struct list_lru shadow_nodes;
void workingset_update_node(struct xa_node *node)
{
+ struct address_space *mapping;
+
/*
* Track non-empty nodes that contain only shadow entries;
* unlink those that contain pages or are being freed.
@@ -442,7 +444,8 @@ void workingset_update_node(struct xa_node *node)
* already where they should be. The list_empty() test is safe
* as node->private_list is protected by the i_pages lock.
*/
- VM_WARN_ON_ONCE(!irqs_disabled()); /* For __inc_lruvec_page_state */
+ mapping = container_of(node->array, struct address_space, i_pages);
+ lockdep_assert_held(&mapping->i_pages.xa_lock);
if (node->count && node->count == node->nr_values) {
if (list_empty(&node->private_list)) {
--
2.35.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH 2/2] mm: workingset: Replace IRQ-off check with a lockdep assert.
@ 2022-03-01 12:21 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 10+ messages in thread
From: Sebastian Andrzej Siewior @ 2022-03-01 12:21 UTC (permalink / raw)
To: cgroups-u79uwXL29TY76Z2rM5mHXA, linux-mm-Bw31MaZKKs3YtjvyW6yDsg
Cc: Andrew Morton, Johannes Weiner, Tejun Heo, Zefan Li,
Thomas Gleixner, Sebastian Andrzej Siewior
Commit
68d48e6a2df57 ("mm: workingset: add vmstat counter for shadow nodes")
introduced an IRQ-off check to ensure that a lock is held which also
disabled interrupts. This does not work the same way on PREEMPT_RT
because none of the locks, that are held, disable interrupts.
Replace this check with a lockdep assert which ensures that the lock is
held.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
mm/workingset.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/workingset.c b/mm/workingset.c
index 2e4fd7c3296fe..8a3828acc0bfd 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -434,6 +434,8 @@ struct list_lru shadow_nodes;
void workingset_update_node(struct xa_node *node)
{
+ struct address_space *mapping;
+
/*
* Track non-empty nodes that contain only shadow entries;
* unlink those that contain pages or are being freed.
@@ -442,7 +444,8 @@ void workingset_update_node(struct xa_node *node)
* already where they should be. The list_empty() test is safe
* as node->private_list is protected by the i_pages lock.
*/
- VM_WARN_ON_ONCE(!irqs_disabled()); /* For __inc_lruvec_page_state */
+ mapping = container_of(node->array, struct address_space, i_pages);
+ lockdep_assert_held(&mapping->i_pages.xa_lock);
if (node->count && node->count == node->nr_values) {
if (list_empty(&node->private_list)) {
--
2.35.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* Re: [PATCH 1/2] cgroup: Use irqsave in cgroup_rstat_flush_locked().
@ 2022-03-02 6:38 ` Tejun Heo
0 siblings, 0 replies; 10+ messages in thread
From: Tejun Heo @ 2022-03-02 6:38 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: cgroups, linux-mm, Andrew Morton, Johannes Weiner, Zefan Li,
Thomas Gleixner
Hello,
On Tue, Mar 01, 2022 at 01:21:42PM +0100, Sebastian Andrzej Siewior wrote:
> All callers of cgroup_rstat_flush_locked() acquire cgroup_rstat_lock
> either with spin_lock_irq() or spin_lock_irqsave().
> cgroup_rstat_flush_locked() itself acquires cgroup_rstat_cpu_lock which
> is a raw_spin_lock. This lock is also acquired in cgroup_rstat_updated()
> in IRQ context and therefore requires _irqsave() locking suffix in
> cgroup_rstat_flush_locked().
> Since there is no difference between spin_lock_t and raw_spin_lock_t
> on !RT lockdep does not complain here. On RT lockdep complains because
> the interrupts were not disabled here and a deadlock is possible.
>
> Acquire the raw_spin_lock_t with disabled interrupts.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Can you please add a comment explaining why irqsave is being used? As it
stands, it just looks spurious.
Thanks.
--
tejun
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH 1/2] cgroup: Use irqsave in cgroup_rstat_flush_locked().
@ 2022-03-02 6:38 ` Tejun Heo
0 siblings, 0 replies; 10+ messages in thread
From: Tejun Heo @ 2022-03-02 6:38 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA, linux-mm-Bw31MaZKKs3YtjvyW6yDsg,
Andrew Morton, Johannes Weiner, Zefan Li, Thomas Gleixner
Hello,
On Tue, Mar 01, 2022 at 01:21:42PM +0100, Sebastian Andrzej Siewior wrote:
> All callers of cgroup_rstat_flush_locked() acquire cgroup_rstat_lock
> either with spin_lock_irq() or spin_lock_irqsave().
> cgroup_rstat_flush_locked() itself acquires cgroup_rstat_cpu_lock which
> is a raw_spin_lock. This lock is also acquired in cgroup_rstat_updated()
> in IRQ context and therefore requires _irqsave() locking suffix in
> cgroup_rstat_flush_locked().
> Since there is no difference between spin_lock_t and raw_spin_lock_t
> on !RT lockdep does not complain here. On RT lockdep complains because
> the interrupts were not disabled here and a deadlock is possible.
>
> Acquire the raw_spin_lock_t with disabled interrupts.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Can you please add a comment explaining why irqsave is being used? As it
stands, it just looks spurious.
Thanks.
--
tejun
^ permalink raw reply [flat|nested] 10+ messages in thread
* [PATCH] cgroup: Add a comment to cgroup_rstat_flush_locked().
@ 2022-03-02 14:46 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 10+ messages in thread
From: Sebastian Andrzej Siewior @ 2022-03-02 14:46 UTC (permalink / raw)
To: Tejun Heo
Cc: cgroups, linux-mm, Andrew Morton, Johannes Weiner, Zefan Li,
Thomas Gleixner
Add a comment why spin_lock_irq() -> raw_spin_lock_irqsave() is needed.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
On 2022-03-01 20:38:51 [-1000], Tejun Heo wrote:
> Hello,
Hello Tejun,
> Can you please add a comment explaining why irqsave is being used? As it
> stands, it just looks spurious.
Something like this?
kernel/cgroup/rstat.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 53b771c20ee50..ba7a660184e41 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -155,6 +155,14 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
struct cgroup *pos = NULL;
unsigned long flags;
+ /*
+ * The _irqsave() is needed because cgroup_rstat_lock is
+ * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
+ * this lock with the _irq() suffix only disables interrupts on
+ * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
+ * interrupts on both configurations. The _irqsave() ensures
+ * that interrupts are always disabled and later restored.
+ */
raw_spin_lock_irqsave(cpu_lock, flags);
while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
struct cgroup_subsys_state *css;
--
2.35.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH] cgroup: Add a comment to cgroup_rstat_flush_locked().
@ 2022-03-02 14:46 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 10+ messages in thread
From: Sebastian Andrzej Siewior @ 2022-03-02 14:46 UTC (permalink / raw)
To: Tejun Heo
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA, linux-mm-Bw31MaZKKs3YtjvyW6yDsg,
Andrew Morton, Johannes Weiner, Zefan Li, Thomas Gleixner
Add a comment why spin_lock_irq() -> raw_spin_lock_irqsave() is needed.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
On 2022-03-01 20:38:51 [-1000], Tejun Heo wrote:
> Hello,
Hello Tejun,
> Can you please add a comment explaining why irqsave is being used? As it
> stands, it just looks spurious.
Something like this?
kernel/cgroup/rstat.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 53b771c20ee50..ba7a660184e41 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -155,6 +155,14 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
struct cgroup *pos = NULL;
unsigned long flags;
+ /*
+ * The _irqsave() is needed because cgroup_rstat_lock is
+ * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
+ * this lock with the _irq() suffix only disables interrupts on
+ * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
+ * interrupts on both configurations. The _irqsave() ensures
+ * that interrupts are always disabled and later restored.
+ */
raw_spin_lock_irqsave(cpu_lock, flags);
while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
struct cgroup_subsys_state *css;
--
2.35.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* Re: [PATCH] cgroup: Add a comment to cgroup_rstat_flush_locked().
@ 2022-03-02 15:47 ` Tejun Heo
0 siblings, 0 replies; 10+ messages in thread
From: Tejun Heo @ 2022-03-02 15:47 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: cgroups, linux-mm, Andrew Morton, Johannes Weiner, Zefan Li,
Thomas Gleixner
On Wed, Mar 02, 2022 at 03:46:16PM +0100, Sebastian Andrzej Siewior wrote:
> Add a comment why spin_lock_irq() -> raw_spin_lock_irqsave() is needed.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> On 2022-03-01 20:38:51 [-1000], Tejun Heo wrote:
> > Hello,
>
> Hello Tejun,
>
> > Can you please add a comment explaining why irqsave is being used? As it
> > stands, it just looks spurious.
>
> Something like this?
Yeah, looks good to me.
Acked-by: Tejun Heo <tj@kernel.org>
Thanks.
--
tejun
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH] cgroup: Add a comment to cgroup_rstat_flush_locked().
@ 2022-03-02 15:47 ` Tejun Heo
0 siblings, 0 replies; 10+ messages in thread
From: Tejun Heo @ 2022-03-02 15:47 UTC (permalink / raw)
To: Sebastian Andrzej Siewior
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA, linux-mm-Bw31MaZKKs3YtjvyW6yDsg,
Andrew Morton, Johannes Weiner, Zefan Li, Thomas Gleixner
On Wed, Mar 02, 2022 at 03:46:16PM +0100, Sebastian Andrzej Siewior wrote:
> Add a comment why spin_lock_irq() -> raw_spin_lock_irqsave() is needed.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> ---
> On 2022-03-01 20:38:51 [-1000], Tejun Heo wrote:
> > Hello,
>
> Hello Tejun,
>
> > Can you please add a comment explaining why irqsave is being used? As it
> > stands, it just looks spurious.
>
> Something like this?
Yeah, looks good to me.
Acked-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Thanks.
--
tejun
^ permalink raw reply [flat|nested] 10+ messages in thread
end of thread, other threads:[~2022-03-02 15:47 UTC | newest]
Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
[not found] <[PATCH 0/2] Correct locking assumption on PREEMPT_RT>
[not found] ` <20220301122143.1521823-1-bigeasy@linutronix.de>
2022-03-01 12:21 ` [PATCH 1/2] cgroup: Use irqsave in cgroup_rstat_flush_locked() Sebastian Andrzej Siewior
2022-03-01 12:21 ` Sebastian Andrzej Siewior
2022-03-02 6:38 ` Tejun Heo
2022-03-02 6:38 ` Tejun Heo
2022-03-02 14:46 ` [PATCH] cgroup: Add a comment to cgroup_rstat_flush_locked() Sebastian Andrzej Siewior
2022-03-02 14:46 ` Sebastian Andrzej Siewior
2022-03-02 15:47 ` Tejun Heo
2022-03-02 15:47 ` Tejun Heo
2022-03-01 12:21 ` [PATCH 2/2] mm: workingset: Replace IRQ-off check with a lockdep assert Sebastian Andrzej Siewior
2022-03-01 12:21 ` Sebastian Andrzej Siewior
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