Re: [patch] spinlocks: remove 'volatile'

[Ralf Baechle <ralf@linux-mips.org>]

On Thu, Jul 06, 2006 at 01:34:14PM -0700, Linus Torvalds wrote:

> On Thu, 6 Jul 2006, Linus Torvalds wrote:
> > 
> > So I _think_ that we should change the "=m" to the much more correct "+m" 
> > at the same time (or before - it's really a bug-fix regardless) as 
> > removing the "volatile".
> 
> Here's a first cut (UNTESTED!) for x86. I didn't check any other 
> architectures, I bet they have similar problems.

I tried the same on MIPS, for lazyness sake at first only in atomic.h.  With
gcc 3.3 the code size is exactly the same with both "=m" and "+m", so I
didn't look into details of the generated code.  With gcc 4.1 "+m" results
in a size increase of about 1K for the ip27_defconfig kernel.  For example:

<unlock_kernel>:
       df830000        ld      v1,0(gp)
       8c620028        lw      v0,40(v1)
       04400014        bltz    v0,a80000000029944c <unlock_kernel+0x5c>
       00000000        nop
       2442ffff        subiu   v0,v0,1
       ac620028        sw      v0,40(v1)	# current->lock_depth
       8c630028        lw      v1,40(v1)	# current->lock_depth
       0461000b        bgez    v1,a80000000029943c <unlock_kernel+0x4c>

The poinless load isn't generated with "=m".  The interesting thing is
that in all the instances of bloat I looked at it was actually happening
not as part of the asm statement itself, so maybe gcc's reload is getting
a little confused.

  Ralf