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

[Nicolas Pitre]

On Tue, 5 Aug 2014, Stephen Boyd wrote:

> 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.

OK I get it now.

> I hoped that the commit text explained this.

I'm possibly not bright enough to get it the first time.

> Honestly it probably isn't a noticeable performance boost either way 
> but I think this is the best we can do.

Sure, agreed.


Nicolas
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