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Date: Sat, 25 Oct 2008 10:22:38 +0530
From: Gautham R Shenoy <ego@...ibm.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] timers: Handle HRTIMER_CB_IRQSAFE_UNLOCKED correctly
from softirq context
On Fri, Oct 24, 2008 at 07:33:03PM -0700, Paul E. McKenney wrote:
> On Fri, Oct 24, 2008 at 03:20:03AM +0530, Gautham R Shenoy wrote:
> > timers: Handle HRTIMER_CB_IRQSAFE_UNLOCKED correctly from softirq context
> >
> > From: Gautham R Shenoy <ego@...ibm.com>
> >
> > While migrating the the CB_IRQSAFE_UNLOCKED timers during a cpu-offline,
> > we queue them on the cb_pending list, so that they won't go
> > stale.
> >
> > Thus, when the callbacks of the timers run from the softirq context,
> > they could run into potential deadlocks, since these callbacks
> > assume that they're running with irq's disabled, thereby annoying
> > lockdep (see below)!
> >
> > Fix this by emulating hardirq context while running these callbacks from
> > the hrtimer softirq.
>
> Good catch!!! One confusion (probably on my part) below.
Hmm.. Let's see...
>
> > diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> > index cdec83e..60aaad6 100644
> > --- a/kernel/hrtimer.c
> > +++ b/kernel/hrtimer.c
> > @@ -1188,6 +1188,7 @@ static void run_hrtimer_pending(struct hrtimer_cpu_base *cpu_base)
> > enum hrtimer_restart (*fn)(struct hrtimer *);
> > struct hrtimer *timer;
> > int restart;
> > + int emulate_hardirq_ctx = 0;
> >
> > timer = list_entry(cpu_base->cb_pending.next,
> > struct hrtimer, cb_entry);
> > @@ -1196,12 +1197,26 @@ static void run_hrtimer_pending(struct hrtimer_cpu_base *cpu_base)
> > timer_stats_account_hrtimer(timer);
> >
> > fn = timer->function;
> > + /*
> > + * A timer might have been added to the cb_pending list
> > + * when it was migrated during a cpu-offline operation.
> > + * Emulate hardirq context for such timers.
> > + */
> > + if (timer->cb_mode == HRTIMER_CB_IRQSAFE_PERCPU ||
> > + timer->cb_mode == HRTIMER_CB_IRQSAFE_UNLOCKED)
> > + emulate_hardirq_ctx = 1;
>
> Is this the case where we need to emulate a hardirq context?
Indeed it is.
>
> > +
> > __remove_hrtimer(timer, timer->base, HRTIMER_STATE_CALLBACK, 0);
> > - spin_unlock_irq(&cpu_base->lock);
^^^^^^^^^^ ----------------------------------------(A)
> > + spin_unlock(&cpu_base->lock);
> >
> > - restart = fn(timer);
> > + if (likely(!emulate_hardirq_ctx)) {
>
> If so, why the "!" above?
Since this function runs from the HRTIMER_SOFTIRQ, it enables interrupts
before calling the hrtimer-callback function. This has been the original
behaviour (see A above)
We would like to retain this behavior for the normal cases where we
don't want to emulate hard-irq context.
So if you see below, all we're doing in the normal case is enabling the
interrupts before calling the callback in the normal case, and
keeping the interrupts disabled when we want to emulate the hard-irq case.
Because HRTIMER_CB_IRQSAFE_UNLOCKED callbacks assume that they run with
interrupts disabled.
>
> Or am I misinterpreting the name of the variable?
>
> Thanx, Paul
>
> > + local_irq_enable();
^^^^^^^^ ----------------------- (B)
> > + restart = fn(timer);
> > + local_irq_disable();
> > + } else
> > + restart = fn(timer);
> >
Probably it may appear as though we're doing something special
for the "!" case, but infact it's the opposite.
Hmmm, how about the following instead ?
Thanks and Regards
gautham.
--------->
timers: Handle HRTIMER_CB_IRQSAFE_UNLOCKED correctly from softirq context
From: Gautham R Shenoy <ego@...ibm.com>
While migrating the the CB_IRQSAFE_UNLOCKED timers during a cpu-offline,
we queue them on the cb_pending list, so that they won't go
stale.
Thus, when the callbacks of the timers run from the softirq context,
they could run into potential deadlocks, since these callbacks
assume that they're running with irq's disabled, thereby annoying
lockdep!
Fix this by emulating hardirq context while running these callbacks from
the hrtimer softirq.
=================================
[ INFO: inconsistent lock state ]
2.6.27 #2
--------------------------------
inconsistent {in-hardirq-W} -> {hardirq-on-W} usage.
ksoftirqd/0/4 [HC0[0]:SC1[1]:HE1:SE0] takes:
(&rq->lock){++..}, at: [<c011db84>] sched_rt_period_timer+0x9e/0x1fc
{in-hardirq-W} state was registered at:
[<c014103c>] __lock_acquire+0x549/0x121e
[<c0107890>] native_sched_clock+0x88/0x99
[<c013aa12>] clocksource_get_next+0x39/0x3f
[<c0139abc>] update_wall_time+0x616/0x7df
[<c0141d6b>] lock_acquire+0x5a/0x74
[<c0121724>] scheduler_tick+0x3a/0x18d
[<c047ed45>] _spin_lock+0x1c/0x45
[<c0121724>] scheduler_tick+0x3a/0x18d
[<c0121724>] scheduler_tick+0x3a/0x18d
[<c012c436>] update_process_times+0x3a/0x44
[<c013c044>] tick_periodic+0x63/0x6d
[<c013c062>] tick_handle_periodic+0x14/0x5e
[<c010568c>] timer_interrupt+0x44/0x4a
[<c0150c9f>] handle_IRQ_event+0x13/0x3d
[<c0151c14>] handle_level_irq+0x79/0xbd
[<c0105634>] do_IRQ+0x69/0x7d
[<c01041e4>] common_interrupt+0x28/0x30
[<c047007b>] aac_probe_one+0x1a3/0x3f3
[<c047ec2d>] _spin_unlock_irqrestore+0x36/0x39
[<c01512b4>] setup_irq+0x1be/0x1f9
[<c065d70b>] start_kernel+0x259/0x2c5
[<ffffffff>] 0xffffffff
irq event stamp: 50102
hardirqs last enabled at (50102): [<c047ebf4>] _spin_unlock_irq+0x20/0x23
hardirqs last disabled at (50101): [<c047edc2>] _spin_lock_irq+0xa/0x4b
softirqs last enabled at (50088): [<c0128ba6>] do_softirq+0x37/0x4d
softirqs last disabled at (50099): [<c0128ba6>] do_softirq+0x37/0x4d
other info that might help us debug this:
no locks held by ksoftirqd/0/4.
stack backtrace:
Pid: 4, comm: ksoftirqd/0 Not tainted 2.6.27 #2
[<c013f6cb>] print_usage_bug+0x13e/0x147
[<c013fef5>] mark_lock+0x493/0x797
[<c01410b1>] __lock_acquire+0x5be/0x121e
[<c0141d6b>] lock_acquire+0x5a/0x74
[<c011db84>] sched_rt_period_timer+0x9e/0x1fc
[<c047ed45>] _spin_lock+0x1c/0x45
[<c011db84>] sched_rt_period_timer+0x9e/0x1fc
[<c011db84>] sched_rt_period_timer+0x9e/0x1fc
[<c01210fd>] finish_task_switch+0x41/0xbd
[<c0107890>] native_sched_clock+0x88/0x99
[<c011dae6>] sched_rt_period_timer+0x0/0x1fc
[<c0136dda>] run_hrtimer_pending+0x54/0xe5
[<c011dae6>] sched_rt_period_timer+0x0/0x1fc
[<c0128afb>] __do_softirq+0x7b/0xef
[<c0128ba6>] do_softirq+0x37/0x4d
[<c0128c12>] ksoftirqd+0x56/0xc5
[<c0128bbc>] ksoftirqd+0x0/0xc5
[<c0134649>] kthread+0x38/0x5d
[<c0134611>] kthread+0x0/0x5d
[<c0104477>] kernel_thread_helper+0x7/0x10
=======================
Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
kernel/hrtimer.c | 17 ++++++++++++++++-
1 files changed, 16 insertions(+), 1 deletions(-)
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index cdec83e..f49bc14 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1188,6 +1188,7 @@ static void run_hrtimer_pending(struct hrtimer_cpu_base *cpu_base)
enum hrtimer_restart (*fn)(struct hrtimer *);
struct hrtimer *timer;
int restart;
+ int emulate_hardirq_ctx = 0;
timer = list_entry(cpu_base->cb_pending.next,
struct hrtimer, cb_entry);
@@ -1196,10 +1197,24 @@ static void run_hrtimer_pending(struct hrtimer_cpu_base *cpu_base)
timer_stats_account_hrtimer(timer);
fn = timer->function;
+ /*
+ * A timer might have been added to the cb_pending list
+ * when it was migrated during a cpu-offline operation.
+ * Emulate hardirq context for such timers.
+ */
+ if (timer->cb_mode == HRTIMER_CB_IRQSAFE_PERCPU ||
+ timer->cb_mode == HRTIMER_CB_IRQSAFE_UNLOCKED)
+ emulate_hardirq_ctx = 1;
+
__remove_hrtimer(timer, timer->base, HRTIMER_STATE_CALLBACK, 0);
spin_unlock_irq(&cpu_base->lock);
- restart = fn(timer);
+ if (unlikely(emulate_hardirq_ctx)) {
+ local_irq_disable();
+ restart = fn(timer);
+ local_irq_enable();
+ } else
+ restart = fn(timer);
spin_lock_irq(&cpu_base->lock);
--
Thanks and Regards
gautham
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