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Re: [PATCH] mutex: make mutex_lock_nested an inline function

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  Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-13 22:40 +0200
    Re: [PATCH] mutex: make mutex_lock_nested an inline function Arnd Bergmann <arnd@arndb.de> - 2015-10-13 23:50 +0200
      Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-14 10:30 +0200
        Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-14 10:40 +0200
          Re: [PATCH] mutex: make mutex_lock_nested an inline function Arnd Bergmann <arnd@arndb.de> - 2015-10-14 11:10 +0200
            Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-14 11:10 +0200
              Re: [PATCH] mutex: make mutex_lock_nested an inline function Mark Brown <broonie@kernel.org> - 2015-10-14 12:10 +0200
        Re: [PATCH] mutex: make mutex_lock_nested an inline function Mark Brown <broonie@kernel.org> - 2015-10-14 12:30 +0200
          Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-14 13:10 +0200
            Re: [PATCH] mutex: make mutex_lock_nested an inline function Mark Brown <broonie@kernel.org> - 2015-10-14 14:40 +0200
              Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-14 15:50 +0200
                Re: [PATCH] mutex: make mutex_lock_nested an inline function Peter Zijlstra <peterz@infradead.org> - 2015-10-14 16:00 +0200
                  Re: [PATCH] mutex: make mutex_lock_nested an inline function Ingo Molnar <mingo@kernel.org> - 2015-10-14 16:00 +0200
                Re: [PATCH] mutex: make mutex_lock_nested an inline function Mark Brown <broonie@kernel.org> - 2015-10-14 16:20 +0200

#1246123 — Re: [PATCH] mutex: make mutex_lock_nested an inline function

On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> The second argument of the mutex_lock_nested() helper is only
> evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> get this build warning for the new regulator_lock_supply
> function:
> 
> drivers/regulator/core.c: In function 'regulator_lock_supply':
> drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> 
> To avoid the warning, this patch changes the definition of
> mutex_lock_nested() to be static inline function rather than
> a macro, which tells gcc that the variable is potentially
> used.

> -# define mutex_lock_nested(lock, subclass) mutex_lock(lock)
> +static inline void mutex_lock_nested(struct mutex *lock, unsigned int subclass)
> +{
> +	return mutex_lock(lock);
> +}

Can you verify that this results in an identical kernel?

Having this a proper argument results in the compiler having to actually
evaluate the expression resulting in @subclass, this might have side
effects and generate code.

A quick grep shows a large amount of trivial code that optimizers will
still happily throw away, but it should be verified that this does not
result in pointless code generation.
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#1246187

On Tuesday 13 October 2015 22:38:12 Peter Zijlstra wrote:
> On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> > The second argument of the mutex_lock_nested() helper is only
> > evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> > get this build warning for the new regulator_lock_supply
> > function:
> > 
> > drivers/regulator/core.c: In function 'regulator_lock_supply':
> > drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> > 
> > To avoid the warning, this patch changes the definition of
> > mutex_lock_nested() to be static inline function rather than
> > a macro, which tells gcc that the variable is potentially
> > used.
> 
> > -# define mutex_lock_nested(lock, subclass) mutex_lock(lock)
> > +static inline void mutex_lock_nested(struct mutex *lock, unsigned int subclass)
> > +{
> > +	return mutex_lock(lock);
> > +}
> 
> Can you verify that this results in an identical kernel?
> 
> Having this a proper argument results in the compiler having to actually
> evaluate the expression resulting in @subclass, this might have side
> effects and generate code.
> 
> A quick grep shows a large amount of trivial code that optimizers will
> still happily throw away, but it should be verified that this does not
> result in pointless code generation.

Indeed, I'm seeing a tiny code growth with ARM multi_v7_defconfig when
my patch is applied, as the image (according to size -A) grows from
13740187 bytes to 13740283, all of it in .text of two drivers (i2c-core
and three files of bluetooth.ko).

--- build/multi_v7_defconfig-before/vmlinux.o.size	2015-10-13 23:11:40.544389776 +0200
+++ build/multi_v7_defconfig/vmlinux.o.size	2015-10-13 23:08:00.151043811 +0200
@@ -1,6 +1,6 @@
 build/multi_v7_defconfig/vmlinux.o  :
 section                                                          size   addr
-.text                                                         8219408      0
+.text                                                         8219504      0
 
--- build/multi_v7_defconfig-before/net/bluetooth/bluetooth.ko.size	2015-10-13 23:11:40.704382038 +0200
+++ build/multi_v7_defconfig/net/bluetooth/bluetooth.ko.size	2015-10-13 23:07:58.639116862 +0200
@@ -1,7 +1,7 @@
 build/multi_v7_defconfig/net/bluetooth/bluetooth.ko  :
 section                       size   addr
 .note.gnu.build-id              36      0
-.text                       241512      0
+.text                       241696      0
 
--- build/multi_v7_defconfig-before/drivers/i2c/i2c-core.o.size	2015-10-13 23:11:40.636385326 +0200
+++ build/multi_v7_defconfig/drivers/i2c/i2c-core.o.size	2015-10-13 23:07:53.403369830 +0200
@@ -1,6 +1,6 @@
 build/multi_v7_defconfig/drivers/i2c/i2c-core.o  :
 section                                                 size   addr
-.text                                                  12112      0
+.text                                                  12208      0

The code in question is 

a)

static ssize_t
i2c_sysfs_delete_device(struct device *dev, struct device_attribute *attr,
                        const char *buf, size_t count)
{
...
        mutex_lock_nested(&adap->userspace_clients_lock,
                          i2c_adapter_depth(adap));
...
}

and

b)

static inline void l2cap_chan_lock(struct l2cap_chan *chan)
{                             
        mutex_lock_nested(&chan->lock, atomic_read(&chan->nesting));
}       

The first one has a small size impact but no performance change as it is only
called during probe/release of i2c modules. The second one adds an extra
pointer access (due to the volatile keyword in atomic_read()) for every
caller of l2cap_chan_lock().

	Arnd
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#1246399

> > On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> > > The second argument of the mutex_lock_nested() helper is only
> > > evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> > > get this build warning for the new regulator_lock_supply
> > > function:
> > > 
> > > drivers/regulator/core.c: In function 'regulator_lock_supply':
> > > drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> > > 
> > > To avoid the warning, this patch changes the definition of
> > > mutex_lock_nested() to be static inline function rather than
> > > a macro, which tells gcc that the variable is potentially
> > > used.

Uuh, I just looked at next and saw this regulator_lock_supply()
function. How is that limited? subclass must be <8 otherwise bad things
happen.
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#1246411

On Wed, Oct 14, 2015 at 10:20:50AM +0200, Peter Zijlstra wrote:
> > > On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> > > > The second argument of the mutex_lock_nested() helper is only
> > > > evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> > > > get this build warning for the new regulator_lock_supply
> > > > function:
> > > > 
> > > > drivers/regulator/core.c: In function 'regulator_lock_supply':
> > > > drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> > > > 
> > > > To avoid the warning, this patch changes the definition of
> > > > mutex_lock_nested() to be static inline function rather than
> > > > a macro, which tells gcc that the variable is potentially
> > > > used.
> 
> Uuh, I just looked at next and saw this regulator_lock_supply()
> function. How is that limited? subclass must be <8 otherwise bad things
> happen.

Also, the function appears unused, just delete it ;-)
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#1246436

On Wednesday 14 October 2015 10:37:07 Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 10:20:50AM +0200, Peter Zijlstra wrote:
> > > > On Tue, Oct 13, 2015 at 10:30:08PM +0200, Arnd Bergmann wrote:
> > > > > The second argument of the mutex_lock_nested() helper is only
> > > > > evaluated if CONFIG_DEBUG_LOCK_ALLOC is set. Otherwise we
> > > > > get this build warning for the new regulator_lock_supply
> > > > > function:
> > > > > 
> > > > > drivers/regulator/core.c: In function 'regulator_lock_supply':
> > > > > drivers/regulator/core.c:142:6: warning: unused variable 'i' [-Wunused-variable]
> > > > > 
> > > > > To avoid the warning, this patch changes the definition of
> > > > > mutex_lock_nested() to be static inline function rather than
> > > > > a macro, which tells gcc that the variable is potentially
> > > > > used.
> > 
> > Uuh, I just looked at next and saw this regulator_lock_supply()
> > function. How is that limited? subclass must be <8 otherwise bad things
> > happen.
> 
> Also, the function appears unused, just delete it 

That was my first suggestion as well when I ran into 

drivers/regulator/core.c:139:13: warning: 'regulator_lock_supply' defined but not used

but apparently this is work-in-progress and the plan is to use it
in 4.4 when the rest of the currently-under-review patches are merged.

	Arnd
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#1246441

On Wed, Oct 14, 2015 at 11:00:20AM +0200, Arnd Bergmann wrote:
> On Wednesday 14 October 2015 10:37:07 Peter Zijlstra wrote:

> > > Uuh, I just looked at next and saw this regulator_lock_supply()
> > > function. How is that limited? subclass must be <8 otherwise bad things
> > > happen.
> > 
> > Also, the function appears unused, just delete it 
> 
> That was my first suggestion as well when I ran into 
> 
> drivers/regulator/core.c:139:13: warning: 'regulator_lock_supply' defined but not used
> 
> but apparently this is work-in-progress and the plan is to use it
> in 4.4 when the rest of the currently-under-review patches are merged.

And here I thought we had a fairly strong rule that we should not merge
unused code :/

In any case, whomever wrote that function had better first explain why
its not broken.

And I'm not too keen on making mutex_lock_nested() an inline due it
being too easy to generate code with it. I'd much rather work around the
occasional warning than have to deal with silent but unintended code
spills.
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#1246479

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On Wed, Oct 14, 2015 at 11:08:15AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 11:00:20AM +0200, Arnd Bergmann wrote:
> > On Wednesday 14 October 2015 10:37:07 Peter Zijlstra wrote:

> > but apparently this is work-in-progress and the plan is to use it
> > in 4.4 when the rest of the currently-under-review patches are merged.

> And here I thought we had a fairly strong rule that we should not merge
> unused code :/

The revisions are very minor (and are sitting in my inbox for me to
look at just now), and keeping things out of tree just makes it more
difficult to get them merged - it's one thing if things are getting in
the way for a long time or have a high cost but that's not the case
here.

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#1246492

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On Wed, Oct 14, 2015 at 10:20:50AM +0200, Peter Zijlstra wrote:

> Uuh, I just looked at next and saw this regulator_lock_supply()
> function. How is that limited? subclass must be <8 otherwise bad things
> happen.

Can we please get some more discoverable documentation of the arbitrary
limits in the lockdep code?  I seem to keep seeing code that bumps into
surprising limits like this and I'm not sure how I'm supposed to know
about them except through finding out after the fact or trawling the
code every time someone touches locking.

I would be very surprised to see a system that pushes over 8 locks,
while there's nothing actually preventing it system design
considerations mean that even four cascaded supplies are pretty
unlikely so we should be fine.  Every time you add a new level of
regulation you're both increasing the load on regulators up the chain
(which means they need to be bigger and more expensive) and except in
the case of a DCDC supplying an LDO (which only works to one level)
you're going to be decreasing the efficiency of the system.  If we get
to that point we can worry about what to do.

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#1246524

On Wed, Oct 14, 2015 at 11:27:06AM +0100, Mark Brown wrote:
> On Wed, Oct 14, 2015 at 10:20:50AM +0200, Peter Zijlstra wrote:
> 
> > Uuh, I just looked at next and saw this regulator_lock_supply()
> > function. How is that limited? subclass must be <8 otherwise bad things
> > happen.
> 
> Can we please get some more discoverable documentation of the arbitrary
> limits in the lockdep code? 

include/linux/lockdep.h:#define MAX_LOCKDEP_SUBCLASSES          8UL

Also, it will give a runtime warn if you ever do hit that.

> I seem to keep seeing code that bumps into
> surprising limits like this and I'm not sure how I'm supposed to know
> about them except through finding out after the fact or trawling the
> code every time someone touches locking.

Not knowing what other limits you've hit, I'm not entirely sure how to
help out there.

> I would be very surprised to see a system that pushes over 8 locks,
> while there's nothing actually preventing it system design
> considerations mean that even four cascaded supplies are pretty
> unlikely so we should be fine.  Every time you add a new level of
> regulation you're both increasing the load on regulators up the chain
> (which means they need to be bigger and more expensive) and except in
> the case of a DCDC supplying an LDO (which only works to one level)
> you're going to be decreasing the efficiency of the system.  If we get
> to that point we can worry about what to do.

OK I suppose.
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#1246626

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On Wed, Oct 14, 2015 at 01:07:17PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 11:27:06AM +0100, Mark Brown wrote:
> > On Wed, Oct 14, 2015 at 10:20:50AM +0200, Peter Zijlstra wrote:

> > > Uuh, I just looked at next and saw this regulator_lock_supply()
> > > function. How is that limited? subclass must be <8 otherwise bad things
> > > happen.

> > Can we please get some more discoverable documentation of the arbitrary
> > limits in the lockdep code? 

> include/linux/lockdep.h:#define MAX_LOCKDEP_SUBCLASSES          8UL

Sure, but I don't really expect to have to trawl the implementation of
an API to find out about this sort of thing (I hadn't even been aware
that the subclasses were required to be small positive integers, never
mind needing to check what the limit was).  I think the main place I'd
have expected to see it was in lockdep-design.txt or somewhere near
that.

> > I seem to keep seeing code that bumps into
> > surprising limits like this and I'm not sure how I'm supposed to know
> > about them except through finding out after the fact or trawling the
> > code every time someone touches locking.

> Not knowing what other limits you've hit, I'm not entirely sure how to
> help out there.

The other big one that came up recently was that lockdep apparently
works out what a class is by looking at the point of allocation which
causes a lot of problems for regmap since it makes all regmap locks look
like a single class.  That's fixed now by explicitly allocating a class
per regmap with some macro magic but it was a bit of a surprise.  The
documentation doesn't make this obvious.

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#1246767

On Wed, Oct 14, 2015 at 01:36:24PM +0100, Mark Brown wrote:

> Sure, but I don't really expect to have to trawl the implementation of
> an API to find out about this sort of thing (I hadn't even been aware

I more like view header files as 'documentation' and c files as
implementation :-)

> that the subclasses were required to be small positive integers, never
> mind needing to check what the limit was).  I think the main place I'd
> have expected to see it was in lockdep-design.txt or somewhere near
> that.

Oh, you're one of them people that actually looks in Documentation/.

> The other big one that came up recently was that lockdep apparently
> works out what a class is by looking at the point of allocation which
> causes a lot of problems for regmap since it makes all regmap locks look
> like a single class.  That's fixed now by explicitly allocating a class
> per regmap with some macro magic but it was a bit of a surprise.  The
> documentation doesn't make this obvious.

Yes, Documentation/locking/lockdep-design.txt needs help -- I'd even
forgotten we had it.

Does the below work for you?

---
 Documentation/locking/lockdep-design.txt | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
index 5001280e9d82..e8e9ad4e6f5e 100644
--- a/Documentation/locking/lockdep-design.txt
+++ b/Documentation/locking/lockdep-design.txt
@@ -24,6 +24,10 @@ a lock-class is used for the first time after bootup it gets registered,
 and all subsequent uses of that lock-class will be attached to this
 lock-class.
 
+A class is typically associated with a lock's initialisation site; although
+its possible to explicitly initialize a lock with a different class key --
+such class keys much come from static storage.
+
 State
 -----
 
@@ -165,6 +169,10 @@ partition.
 The validator treats a lock that is taken in such a nested fashion as a
 separate (sub)class for the purposes of validation.
 
+Since lock classes are associated with static addresses, the size of struct
+lock_class_key determines the amount of sub classes that are possible --
+currently set to 8.
+
 Note: When changing code to use the _nested() primitives, be careful and
 check really thoroughly that the hierarchy is correctly mapped; otherwise
 you can get false positives or false negatives.
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#1246775

On Wed, Oct 14, 2015 at 03:47:21PM +0200, Peter Zijlstra wrote:
> ---
>  Documentation/locking/lockdep-design.txt | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
> index 5001280e9d82..e8e9ad4e6f5e 100644
> --- a/Documentation/locking/lockdep-design.txt
> +++ b/Documentation/locking/lockdep-design.txt
> @@ -24,6 +24,10 @@ a lock-class is used for the first time after bootup it gets registered,
>  and all subsequent uses of that lock-class will be attached to this
>  lock-class.
>  
> +A class is typically associated with a lock's initialisation site; although
> +its possible to explicitly initialize a lock with a different class key --
> +such class keys much come from static storage.

Now if only I could actually type: s/much/must/
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#1246777

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Oct 14, 2015 at 03:47:21PM +0200, Peter Zijlstra wrote:
> > ---
> >  Documentation/locking/lockdep-design.txt | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt
> > index 5001280e9d82..e8e9ad4e6f5e 100644
> > --- a/Documentation/locking/lockdep-design.txt
> > +++ b/Documentation/locking/lockdep-design.txt
> > @@ -24,6 +24,10 @@ a lock-class is used for the first time after bootup it gets registered,
> >  and all subsequent uses of that lock-class will be attached to this
> >  lock-class.
> >  
> > +A class is typically associated with a lock's initialisation site; although
> > +its possible to explicitly initialize a lock with a different class key --
> > +such class keys much come from static storage.
> 
> Now if only I could actually type: s/much/must/

also:

   s/its/it's

:-)

Thanks,

	Ingo
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#1246797

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On Wed, Oct 14, 2015 at 03:47:21PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 01:36:24PM +0100, Mark Brown wrote:

> Does the below work for you?

Yes, that's helpful thanks!

Reviewed-by: Mark Brown <broonie@kernel.org>

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