Date:	Mon, 23 Nov 2015 11:07:42 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
Cc:	Anand Moon <linux.amoon@...il.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Javier Martinez Canillas <javier@....samsung.com>,
	linux-pwm@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Jonathan Richardson <jonathar@...adcom.com>
Subject: Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable

On Sun, Nov 22, 2015 at 09:13:17AM +0900, Krzysztof Kozlowski wrote:
> 2015-11-22 3:14 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
> > Hi Krzysztof,
> >
> > On 21 November 2015 at 18:37, Krzysztof Kozlowski
> > <k.kozlowski@...sung.com> wrote:
> >> 2015-11-21 21:11 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
> >>> hi Krzysztof,
> >>>
> >>> On 21 November 2015 at 15:22, Krzysztof Kozlowski
> >>> <k.kozlowski@...sung.com> wrote:
> >>>> 2015-11-21 18:40 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
> >>>>> hi Krzysztof,
> >>>>>
> >>>>> On 21 November 2015 at 09:56, Krzysztof Kozlowski <k.kozlowski@...sung.com>
> >>>>> wrote:
> >>>>>>
> >>>>>> W dniu 21.11.2015 o 01:59, Anand Moon pisze:
> >>>>>> > Commit d1cd21427747f15920cd726f5f67a07880e7dee4
> >>>>>> > (pwm: Set enable state properly on failed call to enable)
> >>>>>> > introduce double lock of mutex on pwm_enable
> >>>>>> >
> >>>>>> > Changes fix the following debug lock warning.
> >>>>>> >
> >>>>>> > [    2.701720] WARNING: CPU: 3 PID: 0 at kernel/locking/mutex.c:526
> >>>>>> > __mutex_lock_slowpath+0x3bc/0x404()
> >>>>>> > [    2.701731] DEBUG_LOCKS_WARN_ON(in_interrupt())
> >>>>>>
> >>>>>> Hi Anand!
> >>>>>>
> >>>>>> Yeah, I am hitting this as well. Annoying. However your description is
> >>>>>> inaccurate. Double lock of mutex does not cause warnings of sleeping or
> >>>>>> locking in atomic context (if you build with debug sleep atomic you will
> >>>>>> see different warning). More over you are partially reverting the commit
> >>>>>> d1cd21427747 ("pwm: Set enable state properly on failed call to enable")
> >>>>>> without proper explanation:
> >>>>>> 1. why this locking is inappropriate;
> >>>>>> 2. why it is safe to remove the mutex_lock().
> >>>>>>
> >>>>>> Please find the real problem, fix the real problem and throughly explain
> >>>>>> the real issue.
> >>>>>>
> >>>>>> I was suspecting some weird changes in timers. For !irqsafe timer I
> >>>>>> think it is safe to use mutexes... but apparently not. Maybe a work
> >>>>>> should be scheduled from function called by timer?
> >>>>>>
> >>>>>> Warning with debug atomic sleep:
> >>>>>> [   16.047517] BUG: sleeping function called from invalid context at
> >>>>>> ../kernel/locking/mutex.c:617
> >>>>>> [   16.054866] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
> >>>>>> [   16.061402] INFO: lockdep is turned off.
> >>>>>> [   16.065281] Preemption disabled at:[<  (null)>]   (null)
> >>>>>> [   16.070524]
> >>>>>> [   16.072002] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
> >>>>>> 4.4.0-rc1-00040-g28c429565d4f #290
> >>>>>> [   16.080150] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
> >>>>>> [   16.086215] [<c0016780>] (unwind_backtrace) from [<c00132f0>]
> >>>>>> (show_stack+0x10/0x14)
> >>>>>> [   16.093938] [<c00132f0>] (show_stack) from [<c0223ba4>]
> >>>>>> (dump_stack+0x70/0xbc)
> >>>>>> [   16.101122] [<c0223ba4>] (dump_stack) from [<c05ed8e0>]
> >>>>>> (mutex_lock_nested+0x24/0x474)
> >>>>>> [   16.109009] [<c05ed8e0>] (mutex_lock_nested) from [<c0259154>]
> >>>>>> (pwm_enable+0x20/0x7c)
> >>>>>> [   16.116799] [<c0259154>] (pwm_enable) from [<c04400bc>]
> >>>>>> (led_heartbeat_function+0xdc/0x140)
> >>>>>> [   16.125119] [<c04400bc>] (led_heartbeat_function) from [<c00864c8>]
> >>>>>> (call_timer_fn+0x6c/0xf4)
> >>>>>> [   16.133606] [<c00864c8>] (call_timer_fn) from [<c00866f8>]
> >>>>>> (run_timer_softirq+0x1a8/0x230)
> >>>>>> [   16.141846] [<c00866f8>] (run_timer_softirq) from [<c0028e68>]
> >>>>>> (__do_softirq+0x134/0x2c0)
> >>>>>> [   16.149982] [<c0028e68>] (__do_softirq) from [<c0029334>]
> >>>>>> (irq_exit+0xd0/0x114)
> >>>>>> [   16.157255] [<c0029334>] (irq_exit) from [<c0076610>]
> >>>>>> (__handle_domain_irq+0x70/0xe4)
> >>>>>> [   16.165056] [<c0076610>] (__handle_domain_irq) from [<c00094e8>]
> >>>>>> (gic_handle_irq+0x4c/0x94)
> >>>>>> [   16.173376] [<c00094e8>] (gic_handle_irq) from [<c0013db8>]
> >>>>>> (__irq_svc+0x58/0x98)
> >>>>>> [   16.180817] Exception stack(0xc08cdf58 to 0xc08cdfa0)
> >>>>>> [   16.185823] df40:
> >>>>>>    c0010dcc 00000000
> >>>>>> [   16.193997] df60: c08cdfa8 00000000 c05f57c4 00000000 c08ca520
> >>>>>> c08ce4bc c08c7364 c08ce4b4
> >>>>>> [   16.202141] df80: c09121ca 00000000 00000001 c08cdfa8 c0010dcc
> >>>>>> c0010dd0 600f0013 ffffffff
> >>>>>> [   16.210291] [<c0013db8>] (__irq_svc) from [<c0010dd0>]
> >>>>>> (arch_cpu_idle+0x20/0x3c)
> >>>>>> [   16.217661] [<c0010dd0>] (arch_cpu_idle) from [<c0063174>]
> >>>>>> (cpu_startup_entry+0x17c/0x26c)
> >>>>>> [   16.225899] [<c0063174>] (cpu_startup_entry) from [<c0860c40>]
> >>>>>> (start_kernel+0x368/0x3d0)
> >>>>>>
> >>>>>> Best regards,
> >>>>>> Krzysztof
> >>>>>>
> >>>>>>
> >>>>>> > [    2.701737] Modules linked in:
> >>>>>> > [    2.701748] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.4.0-rc1-xu4f
> >>>>>> > #24
> >>>>>> > [    2.701753] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
> >>>>>> > [    2.701787] [<c0015f48>] (unwind_backtrace) from [<c0012d04>]
> >>>>>> > (show_stack+0x10/0x14)
> >>>>>> > [    2.701808] [<c0012d04>] (show_stack) from [<c01f83fc>]
> >>>>>> > (dump_stack+0x84/0xc4)
> >>>>>> > [    2.701824] [<c01f83fc>] (dump_stack) from [<c0023494>]
> >>>>>> > (warn_slowpath_common+0x80/0xb0)
> >>>>>> > [    2.701836] [<c0023494>] (warn_slowpath_common) from [<c00234f4>]
> >>>>>> > (warn_slowpath_fmt+0x30/0x40)
> >>>>>> > [    2.701849] [<c00234f4>] (warn_slowpath_fmt) from [<c056e6b8>]
> >>>>>> > (__mutex_lock_slowpath+0x3bc/0x404)
> >>>>>> > [    2.701864] [<c056e6b8>] (__mutex_lock_slowpath) from [<c056e70c>]
> >>>>>> > (mutex_lock+0xc/0x24)
> >>>>>> > [    2.701884] [<c056e70c>] (mutex_lock) from [<c0228984>]
> >>>>>> > (pwm_enable+0x20/0x7c)
> >>>>>> > [    2.701903] [<c0228984>] (pwm_enable) from [<c03f0000>]
> >>>>>> > (led_heartbeat_function+0x74/0x144)
> >>>>>> > [    2.701919] [<c03f0000>] (led_heartbeat_function) from [<c0074368>]
> >>>>>> > (call_timer_fn+0x24/0x98)
> >>>>>> > [    2.701932] [<c0074368>] (call_timer_fn) from [<c007453c>]
> >>>>>> > (run_timer_softirq+0x160/0x21c)
> >>>>>> > [    2.701946] [<c007453c>] (run_timer_softirq) from [<c0026e10>]
> >>>>>> > (__do_softirq+0x110/0x228)
> >>>>>> > [    2.701959] [<c0026e10>] (__do_softirq) from [<c00271c8>]
> >>>>>> > (irq_exit+0xc0/0xfc)
> >>>>>> > [    2.701976] [<c00271c8>] (irq_exit) from [<c0065180>]
> >>>>>> > (__handle_domain_irq+0x80/0xec)
> >>>>>> > [    2.701990] [<c0065180>] (__handle_domain_irq) from [<c0009494>]
> >>>>>> > (gic_handle_irq+0x54/0x94)
> >>>>>> > [    2.702001] [<c0009494>] (gic_handle_irq) from [<c0013794>]
> >>>>>> > (__irq_svc+0x54/0x90)
> >>>>>> > [    2.702008] Exception stack(0xee8bdf88 to 0xee8bdfd0)
> >>>>>> > [    2.702019] df80:                   00000001 00000000 00000000
> >>>>>> > c001b720 ee8bc000 c0573354
> >>>>>> > [    2.702031] dfa0: 00000000 00000000 ee8bdfe0 c07f92e4 c08004b4
> >>>>>> > c08004bc f0806640 ee8bdfd8
> >>>>>> > [    2.702039] dfc0: c0010180 c0010184 60000013 ffffffff
> >>>>>> > [    2.702053] [<c0013794>] (__irq_svc) from [<c0010184>]
> >>>>>> > (arch_cpu_idle+0x38/0x3c)
> >>>>>> > [    2.702073] [<c0010184>] (arch_cpu_idle) from [<c0058ed4>]
> >>>>>> > (cpu_startup_entry+0x1ec/0x270)
> >>>>>> > [    2.702086] [<c0058ed4>] (cpu_startup_entry) from [<4000956c>]
> >>>>>> > (0x4000956c)
> >>>>>> > [    2.702093] ---[ end trace 539af70491f4f1a9 ]---
> >>>>>> >
> >>>>>> > Signed-off-by: Anand Moon <linux.amoon@...il.com>
> >>>>>> > ---
> >>>>>> >  drivers/pwm/core.c | 4 ----
> >>>>>> >  1 file changed, 4 deletions(-)
> >>>>>> >
> >>>>>> > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> >>>>>> > index d24ca5f..b8f035a 100644
> >>>>>> > --- a/drivers/pwm/core.c
> >>>>>> > +++ b/drivers/pwm/core.c
> >>>>>> > @@ -506,16 +506,12 @@ int pwm_enable(struct pwm_device *pwm)
> >>>>>> >       if (!pwm)
> >>>>>> >               return -EINVAL;
> >>>>>> >
> >>>>>> > -     mutex_lock(&pwm->lock);
> >>>>>> > -
> >>>>>> >       if (!test_and_set_bit(PWMF_ENABLED, &pwm->flags)) {
> >>>>>> >               err = pwm->chip->ops->enable(pwm->chip, pwm);
> >>>>>> >               if (err)
> >>>>>> >                       clear_bit(PWMF_ENABLED, &pwm->flags);
> >>>>>> >       }
> >>>>>> >
> >>>>>> > -     mutex_unlock(&pwm->lock);
> >>>>>> > -
> >>>>>> >       return err;
> >>>>>> >  }
> >>>>>> >  EXPORT_SYMBOL_GPL(pwm_enable);
> >>>>>> >
> >>>>>
> >>>>>
> >>>>> Adding Jonathan Richardson.
> >>>>>
> >>>>> Yes I am
> >>>>>
> >>>>> aware I am reverting some part of the d1cd21427747 ("pwm: Set enable state
> >>>>> properly on failed call to enable")
> >>>>>
> >>>>> Please take a look at this below commit.
> >>>>>
> >>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/pwm/core.c?id=d1cd21427747f15920cd726f5f67a07880e7dee4
> >>>>>
> >>>>> Actually reverting this change it work fine.
> >>>>>
> >>>>> This changes introduce the new mutex lock pwm->lock to protect enabled bit
> >>>>> by drivers while setting polarity.
> >>>>>
> >>>>> Well pwm_set_polarity already acquires the pwm->lock and calls
> >>>>> pwm_is_enabled function.
> >>>>> Again within pwm_is_enabled we are trying to acquire the same mutex lock.
> >>>>
> >>>> You are describing a lockdown by trying to acquire the same mutex twice.
> >>>>
> >>>> However pwm_is_enabled() does not acquire mutex.
> >>>>
> >>>> Again, please look at generated warnings:
> >>>> 1. BUG: sleeping function called from invalid context
> >>>> 2. DEBUG_LOCKS_WARN_ON(in_interrupt())
> >>>>
> >>>> They are not related anyhow to what you described (acquiring already
> >>>> locked mutex).
> >>>>
> >>>> Best regards,
> >>>> Krzysztof
> >>>
> >>> My last reply mail went in HTML format so resend this.
> >>>
> >>> First it was a typo on my part.
> >>> It not pwm_is_enabled function its pwm_enabled.
> >>
> >> There is no such function as "pwm_enabled".
> >>
> >> Sorry, I don't get your point.
> >>
> >> Instead of pasting some commit use a descriptive way to show the calls
> >> leading lockdown. But please use real function names.
> >>
> >> Best regards,
> >> Krzysztof
> >
> > Earlier my assumption of double mutex lock up totally rubbish.
> >
> > After reverting my changes and building image with CONFIG_DEBUG_ATOMIC_SLEEP.
> >
> > [  390.415370] BUG: sleeping function called from invalid context at
> > kernel/locking/mutex.c:97
> > [  390.422274] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/1
> > [  390.428831] Preemption disabled at:[<  (null)>]   (null)
> > [  391.970352] BUG: sleeping function called from invalid context at
> > kernel/locking/mutex.c:97
> > [  391.977251] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/1
> > [  391.983814] Preemption disabled at:[<  (null)>]   (null)
> > [  393.520376] BUG: sleeping function called from invalid context at
> > kernel/locking/mutex.c:97
> > [  393.527312] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/1
> > [  393.533925] Preemption disabled at:[<  (null)>]   (null)
> 
> Yes, now you pasted the same warning I did...
> 
> This is still the same issue. I already wrote it:
> > 1. BUG: sleeping function called from invalid context
> > 2. DEBUG_LOCKS_WARN_ON(in_interrupt())
> 
> We can repeat it many times but that won't change anything...

This looks like you're simply running the leds-pwm driver with a PWM
that isn't properly marked as potentially sleeping. Unfortunately the
introduction of the mutex in d1cd21427747 ("pwm: Set enable state
properly on failed call to enable") effectively makes all PWM drivers
potentially sleeping. That in turn makes the .can_sleep field obsolete
since all drivers can now sleep.

Any objections to simply removing it and make all users use a workqueue
or some such if they need to control a PWM as a result of an interrupt
trigger?

Thierry

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