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Message-ID: <CAJKOXPdejej2eo4M3fuV9jn7sMMwi7zADsvmvP0n60hW9ER_oQ@mail.gmail.com>
Date: Sat, 21 Nov 2015 18:52:15 +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
2015-11-21 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
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