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

Randy Dunlap
2008-01-15T21:09:25Z

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@infradead.org>:
> > > > 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