Re: [PATCH 1/2] Massive code cleanup of sys_msync()

[Randy Dunlap]

On Tue, 15 Jan 2008 14:46:57 -0600 Matt Mackall wrote:

> 
> On Tue, 2008-01-15 at 11:10 -0800, Randy Dunlap wrote:
> > On Tue, 15 Jan 2008 22:02:54 +0300 Anton Salikhmetov wrote:
> > 
> > > 2008/1/15, Christoph Hellwig <hch@xxxxxxxxxxxxx>:
> > > > On Tue, Jan 15, 2008 at 07:02:44PM +0300, Anton Salikhmetov wrote:
> > 
> > > > > @@ -33,71 +34,65 @@ asmlinkage long sys_msync(unsigned long start, size_t len, int flags)
> > > > >       unsigned long end;
> > > > >       struct mm_struct *mm = current->mm;
> > > > >       struct vm_area_struct *vma;
> > > > > -     int unmapped_error = 0;
> > > > > -     int error = -EINVAL;
> > > > > +     int error = 0, unmapped_error = 0;
> > > > >
> > > > >       if (flags & ~(MS_ASYNC | MS_INVALIDATE | MS_SYNC))
> > > > > -             goto out;
> > > > > +             return -EINVAL;
> > > > >       if (start & ~PAGE_MASK)
> > > > > -             goto out;
> > > > > +             return -EINVAL;
> > > >
> > > > The goto out for a simple return style is used quite commonly in kernel
> > > > code to have a single return statement which makes code maintaince, e.g.
> > > > adding locks or allocations simpler.  Not sure that getting rid of it
> > > > makes a lot of sense.
> > > 
> > > Sorry, I can't agree. That's what is written in the CodingStyle document:
> > > 
> > > The goto statement comes in handy when a function exits from multiple
> > > locations and some common work such as cleanup has to be done.
> > 
> > CodingStyle does not try to cover Everything.  Nor do we want it to.
> > 
> > At any rate, there is a desire for functions to have a single point
> > of return, regardless of the amount of cleanup to be done, so I agree
> > with Christoph's comments.
> 
> When we're not cleaning up resources, the main advantage of having a
> single point of return is that you can trace backwards from the return
> point through the function's logic. But that advantage flies right out
> the window when you use gotos. You still have to figure out how you got
> to the return statement by tracing back and looking at all the possible
> gotos. And the "goto out" style adds bulk and non-negligible complexity
> when we've got to search back for what the last explicitly set value of
> "ret" or "error" or whatever the function in question is using was.
> Sometimes people get this wrong ("retval is already -EINVAL, so I don't
> need to explicitly set it"), and create bugs.
> 
> So I think if we're not actually going to use "structured
> programming" (no gotos) or "stack cleanup" styles, the single return
> point style is more trouble than it's worth.
> 
> A lesser advantage of the single return point is that you can set a
> breakpoint or put a printk at the end of a function. But I don't think
> that's much justification.

OTOH, I think that those are fine reasons for it.

---
~Randy
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