Message-ID: <CAJKOXPdvA4ovVDFyOgaagT+0sOpkUArk9FzWB_Z7iptRCsSAcA@mail.gmail.com>
Date:	Sun, 22 Nov 2015 09:13:17 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Anand Moon <linux.amoon@...il.com>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Thierry Reding <thierry.reding@...il.com>,
	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

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...

Best regards,
Krzysztof
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