Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Mar 19, 2010 at 12:00:41PM -0400, Jason Baron wrote:
> On Fri, Mar 19, 2010 at 08:56:00AM -0400, Mathieu Desnoyers wrote:
> > * Frederic Weisbecker (fweisbec@gmail.com) wrote:
> > > On Thu, Mar 18, 2010 at 10:40:42PM -0400, Mathieu Desnoyers wrote:
> > > > Well, the use-case that drove the asm goto implementation _is_ the tracepoints.
> > > > ;)
> > > > 
> > > > > 
> > > > > But, looking at __DO_TRACE:
> > > > > 
> > > > > 	if (it_func) {						\
> > > > > 		do {						\
> > > > > 			((void(*)(proto))(*it_func))(args);	\
> > > > > 		} while (*(++it_func));				\
> > > > > 	}
> > > > > 
> > > > > I would expect the compiler not to load the parameters in the stack
> > > > > before first checking the branch.
> > > > 
> > > > Note that you have to put that in its full context. It's a macro expanded within
> > > > a static inline function. The initial parameters are passed to the static
> > > > inline, not directly as "args" here. So parameters with side-effects have to be
> > > > evaluated before their result can be passed to the static inline function, so in
> > > > that sense their evaluation cannot be moved into the conditional branch.
> > > 
> > > 
> > > Evaluation yeah, I agree. A function passed as an argument is
> > > going to be evaluated indeed, or whatever thing that has a side effect.
> > > But there is nothing here that need to setup the parameters to the stack
> > > right before the true tracepoint call, not until we passed the branch check
> > > once.
> > > 
> > >  
> > > > > So, the fact that parameters are not loaded before we know we'll call
> > > > > the tracepoint is something we already have or is it something that the jump
> > > > > label brings in the package somehow?
> > > > 
> > > > It's standard compiler optimization behavior.
> > > 
> > > 
> > > Sure. My doubt is: currently with the upstream version, does the
> > > compiler tend to load the parameters to the stack before the branch is
> > > checked? Or is this a magic that jmp labels bring for whatever reason?
> > 
> > Even without the static jump patching, the compiler takes care of putting the
> > stack setup after the branch is checked. That worked with a standard test on a
> > variable, with immediate values and should still work with asm gotos.
> 
> right. stack setup happens after the branch is checked for asm gotos as
> well. However, as mentioned functions as parameters, which have side-effects
> need to be evaluated in the off case, there's nothing to be done about
> that as its a correctness issue.
> 
> Hoever, constructs like a->b, do evaluated even in the disabled case.
> This could be solved via macros, see my proposed patch set: 
> http://marc.info/?l=linux-kernel&m=124276710606872&w=2
> 
> However, the conclusion of the thread was that this should be done in
> the compiler, and as such I filed a bug with gcc about this issue.
> 
> I'll re-post an updated jump label series shortly.


Ok, thanks guys for these informations.