Re: [PATCH v2] irqchip: gic: Allow gic_arch_extn hooks to call into scheduler

[Stephen Boyd]

On 08/05/14 12:50, Nicolas Pitre wrote:
> On Tue, 5 Aug 2014, Stephen Boyd wrote:
>
>> +nico (sorry dropped CC for v2)
>>
>> On 08/04/14 16:27, Stephen Boyd wrote:
>>> Commit 1a6b69b6548c (ARM: gic: add CPU migration support,
>>> 2012-04-12) introduced an acquisition of the irq_controller_lock
>>> in gic_raise_softirq() which can lead to a spinlock recursion if
>>> the gic_arch_extn hooks call into the scheduler (via complete()
>>> or wake_up(), etc.). This happens because gic_arch_extn hooks are
>>> normally called with the irq_controller_lock held and calling
>>> into the scheduler may cause us to call smp_send_reschedule()
>>> which will grab the irq_controller_lock again. Here's an example
>>> from a vendor kernel (note that the gic_arch_extn hook code here
>>> isn't actually in mainline):
>>>
>>> BUG: spinlock recursion on CPU#0, swapper/0/1
>>>  lock: irq_controller_lock+0x0/0x18, .magic: dead4ead, .owner: sw
>>> er_cpu: 0
>>> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.14.10-00430-g3d433c4e
>>>
>>> Call trace:
>>> [<ffffffc000087e1c>] dump_backtrace+0x0/0x140
>>> [<ffffffc000087f6c>] show_stack+0x10/0x1c
>>> [<ffffffc00064732c>] dump_stack+0x74/0xc4
>>> [<ffffffc0006446c0>] spin_dump+0x78/0x88
>>> [<ffffffc0006446f4>] spin_bug+0x24/0x34
>>> [<ffffffc0000d47d0>] do_raw_spin_lock+0x58/0x148
>>> [<ffffffc00064d398>] _raw_spin_lock_irqsave+0x24/0x38
>>> [<ffffffc0002c9d7c>] gic_raise_softirq+0x2c/0xbc
>>> [<ffffffc00008daa4>] smp_send_reschedule+0x34/0x40
>>> [<ffffffc0000c1e94>] try_to_wake_up+0x224/0x288
>>> [<ffffffc0000c1f4c>] default_wake_function+0xc/0x18
>>> [<ffffffc0000ceef0>] __wake_up_common+0x50/0x8c
>>> [<ffffffc0000cef3c>] __wake_up_locked+0x10/0x1c
>>> [<ffffffc0000cf734>] complete+0x3c/0x5c
>>> [<ffffffc0002f0e78>] msm_mpm_enable_irq_exclusive+0x1b8/0x1c8
>>> [<ffffffc0002f0f58>] __msm_mpm_enable_irq+0x4c/0x7c
>>> [<ffffffc0002f0f94>] msm_mpm_enable_irq+0xc/0x18
>>> [<ffffffc0002c9bb0>] gic_unmask_irq+0x40/0x7c
>>> [<ffffffc0000de5f4>] irq_enable+0x2c/0x48
>>> [<ffffffc0000de65c>] irq_startup+0x4c/0x74
>>> [<ffffffc0000dd2fc>] __setup_irq+0x264/0x3f0
>>> [<ffffffc0000dd5e0>] request_threaded_irq+0xcc/0x11c
>>> [<ffffffc0000df254>] devm_request_threaded_irq+0x68/0xb4
>>> [<ffffffc000471520>] msm_iommu_ctx_probe+0x124/0x2d4
>>> [<ffffffc000337374>] platform_drv_probe+0x20/0x54
>>> [<ffffffc00033598c>] driver_probe_device+0x158/0x340
>>> [<ffffffc000335c20>] __driver_attach+0x60/0x90
>>> [<ffffffc000333c9c>] bus_for_each_dev+0x6c/0x8c
>>> [<ffffffc000335304>] driver_attach+0x1c/0x28
>>> [<ffffffc000334f14>] bus_add_driver+0x120/0x204
>>> [<ffffffc0003362e4>] driver_register+0xbc/0x10c
>>> [<ffffffc000337348>] __platform_driver_register+0x5c/0x68
>>> [<ffffffc00094c478>] msm_iommu_driver_init+0x54/0x7c
>>> [<ffffffc0000813ec>] do_one_initcall+0xa4/0x130
>>> [<ffffffc00091d928>] kernel_init_freeable+0x138/0x1dc
>>> [<ffffffc000642578>] kernel_init+0xc/0xd4
>>>
>>> We really just want to synchronize the sending of an SGI with the
>>> update of the gic_cpu_map[], so introduce a new SGI lock that we
>>> can use to synchronize the two code paths. Three main events are
>>> happening that we have to consider:
>>>
>>> 	1. We're updating the gic_cpu_mask to point to an
>>> 	incoming CPU
>>>
>>> 	2. We're (potentially) sending an SGI to the outgoing CPU
>>>
>>> 	3. We're redirecting any pending SGIs for the outgoing
>>> 	CPU to the incoming CPU.
>>>
>>> Events 1 and 3 are already ordered within the same CPU by means
>>> of program order and use of I/O accessors. Events 1 and 2 don't
>>> need to be ordered, but events 2 and 3 do because any SGIs for
>>> the outgoing CPU need to be pending before we can redirect them.
>>> Synchronize by acquiring a new lock around event 2 and before
>>> event 3. Use smp_mb__after_unlock_lock() before event 3 to ensure
>>> that event 1 is seen before event 3 on other CPUs that may be
>>> executing event 2.
>>>
>>> Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
>>> ---
>>>
>>> Changes since v1:
>>>  * Move gic_sgi_lock definition below gic_cpu_map[]
>>>  * Just use spinlock for synchronization instead of over the map update
>>>
>>>  drivers/irqchip/irq-gic.c | 16 ++++++++++++++--
>>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
>>> index 7c131cf7cc13..00bac4627d2e 100644
>>> --- a/drivers/irqchip/irq-gic.c
>>> +++ b/drivers/irqchip/irq-gic.c
>>> @@ -81,6 +81,9 @@ static DEFINE_RAW_SPINLOCK(irq_controller_lock);
>>>  #define NR_GIC_CPU_IF 8
>>>  static u8 gic_cpu_map[NR_GIC_CPU_IF] __read_mostly;
>>>  
>>> +/* Synchronize switching CPU interface and sending SGIs */
>>> +static DEFINE_RAW_SPINLOCK(gic_sgi_lock);
>> This also needs an #ifdef CONFIG_SMP
>>
>>> +
>>>  /*
>>>   * Supported arch specific GIC irq extension.
>>>   * Default make them NULL.
>>> @@ -658,7 +661,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>>>  	int cpu;
>>>  	unsigned long flags, map = 0;
>>>  
>>> -	raw_spin_lock_irqsave(&irq_controller_lock, flags);
>>> +	raw_spin_lock_irqsave(&gic_sgi_lock, flags);
>>>  
>>>  	/* Convert our logical CPU mask into a physical one. */
>>>  	for_each_cpu(cpu, mask)
>>> @@ -673,7 +676,7 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>>>  	/* this always happens on GIC0 */
>>>  	writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
>>>  
>>> -	raw_spin_unlock_irqrestore(&irq_controller_lock, flags);
>>> +	raw_spin_unlock_irqrestore(&gic_sgi_lock, flags);
>>>  }
>>>  #endif
>>>  
>>> @@ -764,6 +767,15 @@ void gic_migrate_target(unsigned int new_cpu_id)
>>>  
>>>  	raw_spin_unlock(&irq_controller_lock);
>>>  
>>> +	raw_spin_lock(&gic_sgi_lock);
>>> +	/*
>>> +	 * Ensure that the gic_cpu_map update above is seen in
>>> +	 * gic_raise_softirq() before we redirect any pending SGIs that
>>> +	 * may have been raised for the outgoing CPU (cur_cpu_id)
>>> +	 */
>>> +	smp_mb__after_unlock_lock();
>>> +	raw_spin_unlock(&gic_sgi_lock);
> I just can't see what protection the above hunk gives you.  I'd simply 
> add the extra lock inside the irq_controller_lock area.

It allows us to synchronize with another CPU that may be inside
gic_raise_softirq(). If the other CPU was in that function then this CPU
would wait until it was done sending the IPI to continue along and
reroute them. If the other CPU was just about to grab the sgi lock then
we would guarantee that the CPU would see the new gic_cpu_map value and
thus any redirection is not necessary. I hoped that the commit text
explained this. Honestly it probably isn't a noticeable performance
boost either way but I think this is the best we can do.

>
>>>  	/*
>>>  	 * Now let's migrate and clear any potential SGIs that might be
>>>  	 * pending for us (cur_cpu_id).  Since GIC_DIST_SGI_PENDING_SET
>> I also wonder if we should do something like this:
>>
>> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
>> index ea3df28043f2..1655c9bcb633 100644
>> --- a/drivers/irqchip/irq-gic.c
>> +++ b/drivers/irqchip/irq-gic.c
>> @@ -663,7 +663,8 @@ static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>>         int cpu;
>>         unsigned long flags, map = 0;
>>
>> -       raw_spin_lock_irqsave(&gic_sgi_lock, flags);
>> +       if (IS_ENABLED(CONFIG_BL_SWITCHER))
>> +               raw_spin_lock_irqsave(&gic_sgi_lock, flags);
> Absolutely.  That has been nagging me for a while but I never came 
> around to make a patch.
>
> If we were certain no b.L platform would ever need the arch hooks, that 
> could even be all that is needed with the existing lock to fix your 
> initial problem.  The switcher code is not meant to be widely used once 
> the scheduler work is in place so I'd lean towards keeping things simple 
> unless a more "complete" solution is actually needed, which in your case 
> is not.

I'm fine with just throwing in the IS_ENABLED stuff and being done, but
I was trying to make the proper fix in case someone else ran into this
issue. Otherwise it's mostly a hack to avoid the scenario. We don't plan
to use bL switcher code and this gic_arch_extn together as far as I know.

-- 
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