From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 15 Jan 2008 13:06:27 -0800 From: Randy Dunlap Subject: Re: [PATCH 1/2] Massive code cleanup of sys_msync() Message-Id: <20080115130627.13b64688.randy.dunlap@oracle.com> In-Reply-To: <1200430017.19146.61.camel@cinder.waste.org> References: <12004129652397-git-send-email-salikhmetov@gmail.com> <12004129734126-git-send-email-salikhmetov@gmail.com> <20080115175705.GA21557@infradead.org> <4df4ef0c0801151102l4d72b6b5j702e21beb1ebe459@mail.gmail.com> <20080115111018.1e27a229.randy.dunlap@oracle.com> <1200430017.19146.61.camel@cinder.waste.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Matt Mackall Cc: Anton Salikhmetov , Christoph Hellwig , linux-mm@kvack.org, jakob@unthought.net, linux-kernel@vger.kernel.org, valdis.kletnieks@vt.edu, riel@redhat.com, ksm@42.dk, staubach@redhat.com, jesper.juhl@gmail.com, torvalds@linux-foundation.org, a.p.zijlstra@chello.nl, akpm@linux-foundation.org, protasnb@gmail.com, miklos@szeredi.hu List-ID: 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 : > > > > 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org