linux-kernel - Re: [PATCH] mutex: make mutex_lock

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Date:	Tue, 13 Oct 2015 23:46:35 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Mark Brown <broonie@...nel.org>,
	Liam Girdwood <lgirdwood@...il.com>, akpm@...ux-foundation.org
Subject: Re: [PATCH] mutex: make mutex_lock_nested an inline function

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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