[PATCH] mutex: make mutex_lock_nested an inline function

[PATCH] mutex: make mutex_lock_nested an inline function

[Arnd Bergmann]

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9f01cd4a915 ("regulator: core: introduce function to lock regulators and its supplies")
---
Second try, this time fixing the root of the problem rather than the code
that triggered it, as suggested by Mark Brown.

I guess this means the fix should now probably go through Ingo's sched-locking
branch or through Andrew's mm patches. The warning currently only shows
up in Linux-next because of Mark's regulator branch that is destined for 4.4,
so it's not needed for 4.3 but also harmless.

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 2cb7531e7d7a..9494dafd7a9c 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -158,7 +158,10 @@ extern void mutex_lock(struct mutex *lock);
 extern int __must_check mutex_lock_interruptible(struct mutex *lock);
 extern int __must_check mutex_lock_killable(struct mutex *lock);
 
-# 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);
+}
 # define mutex_lock_interruptible_nested(lock, subclass) mutex_lock_interruptible(lock)
 # define mutex_lock_killable_nested(lock, subclass) mutex_lock_killable(lock)
 # define mutex_lock_nest_lock(lock, nest_lock) mutex_lock(lock)

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

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.

[PATCH] mutex: make mutex_lock_nested an inline function

[Arnd Bergmann]

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

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

> > 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.

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

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 ;-)

[PATCH] mutex: make mutex_lock_nested an inline function

[Arnd Bergmann]

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

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

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.

[PATCH] mutex: make mutex_lock_nested an inline function

[Mark Brown]

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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20151014/e475a529/attachment.sig>

[PATCH] mutex: make mutex_lock_nested an inline function

[Mark Brown]

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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20151014/2eaedd02/attachment.sig>

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

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.

[PATCH] mutex: make mutex_lock_nested an inline function

[Mark Brown]

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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20151014/74474f5e/attachment.sig>

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

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.

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

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/

[PATCH] mutex: make mutex_lock_nested an inline function

[Ingo Molnar]

* 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

[PATCH] mutex: make mutex_lock_nested an inline function

[Mark Brown]

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>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20151014/f9e33bbc/attachment.sig>

[PATCH] mutex: make mutex_lock_nested an inline function

[Arnd Bergmann]

On Tuesday 13 October 2015 23:46:35 Arnd Bergmann wrote:
> On Tuesday 13 October 2015 22:38:12 Peter Zijlstra wrote:
> > 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).


Did we actually reach any conclusion here? We still get the warnings
in the regulator code in linux-next, and I'd like to see either this
patch ("mutex: make mutex_lock_nested an inline function") or
"regulator: core: avoid unused variable warning" get merged.

We could also remove the two functions again, as they are still
unused and we are getting closer to the merge window.

	Arnd

> --- 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().
> 

[PATCH] mutex: make mutex_lock_nested an inline function

[Peter Zijlstra]

On Thu, Oct 22, 2015 at 05:02:45PM +0200, Arnd Bergmann wrote:
> On Tuesday 13 October 2015 23:46:35 Arnd Bergmann wrote:
> > On Tuesday 13 October 2015 22:38:12 Peter Zijlstra wrote:
> > > 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).
> 
> 
> Did we actually reach any conclusion here? We still get the warnings
> in the regulator code in linux-next, and I'd like to see either this
> patch ("mutex: make mutex_lock_nested an inline function") or
> "regulator: core: avoid unused variable warning" get merged.
> 
> We could also remove the two functions again, as they are still
> unused and we are getting closer to the merge window.

Hmm, I was sure I send a reply, but I cannot even find it in my own sent
folder so who knows.

My current preference is to keep the thing a macro and work around it in
the usage site because while these warns are annoying, they're at least
visible. Whereas, with an inline, code bloat is entirely silent. Even if
the sites you found are harmless, there's no saying what the future will
bring etc..

[PATCH] mutex: make mutex_lock_nested an inline function

[Russell King - ARM Linux]

On Thu, Oct 22, 2015 at 05:09:59PM +0200, Peter Zijlstra wrote:
> Hmm, I was sure I send a reply, but I cannot even find it in my own sent
> folder so who knows.
> 
> My current preference is to keep the thing a macro and work around it in
> the usage site because while these warns are annoying, they're at least
> visible. Whereas, with an inline, code bloat is entirely silent. Even if
> the sites you found are harmless, there's no saying what the future will
> bring etc..

I agree - we've got way too many inline functions already.  My biggest
annoyance in that respect is the asm-generic dma_map_single()
implementation that we're now forced to use on ARM, which results in
quite a large chunk of code at every callsite.

The problem there is that when you have drivers which do something like:

	dma = dma_map_single(dev, page_address(page), size, dir);

you end up with code which converts the struct page to a virtual address,
and then you end up with code in the dma_map_single() inline function
which then converts it back to a struct page + offset - none of which,
with modern ARM kernels, the compiler has a hope in hell of optimising.

So we end up with all that junk at every single dma_map_single() callsite.
If dma_map_single() were a library function, it would be a lot smaller
since we'd only have one copy of the complex virt->struct page conversion.

-- 
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

[PATCH] mutex: make mutex_lock_nested an inline function

[Ingo Molnar]

* Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:

> On Thu, Oct 22, 2015 at 05:09:59PM +0200, Peter Zijlstra wrote:
> > Hmm, I was sure I send a reply, but I cannot even find it in my own sent
> > folder so who knows.
> > 
> > My current preference is to keep the thing a macro and work around it in
> > the usage site because while these warns are annoying, they're at least
> > visible. Whereas, with an inline, code bloat is entirely silent. Even if
> > the sites you found are harmless, there's no saying what the future will
> > bring etc..
> 
> I agree - we've got way too many inline functions already.  My biggest annoyance 
> in that respect is the asm-generic dma_map_single() implementation that we're 
> now forced to use on ARM, which results in quite a large chunk of code at every 
> callsite.
> 
> The problem there is that when you have drivers which do something like:
> 
> 	dma = dma_map_single(dev, page_address(page), size, dir);
> 
> you end up with code which converts the struct page to a virtual address, and 
> then you end up with code in the dma_map_single() inline function which then 
> converts it back to a struct page + offset - none of which, with modern ARM 
> kernels, the compiler has a hope in hell of optimising.
> 
> So we end up with all that junk at every single dma_map_single() callsite. If 
> dma_map_single() were a library function, it would be a lot smaller since we'd 
> only have one copy of the complex virt->struct page conversion.

Should be pretty easy to fix, once you know which inline functions hurt.

Thanks,

	Ingo