linux-kernel - Re: [PATCH 2/2] Introduce __cond_lock

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Date:   Fri, 22 Dec 2017 04:31:12 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        linux-mm@...ck.org, Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 2/2] Introduce __cond_lock_err

On Thu, Dec 21, 2017 at 08:21:20PM -0800, Josh Triplett wrote:
> On Thu, Dec 21, 2017 at 05:10:00PM -0800, Matthew Wilcox wrote:
> > Yes, but this define is only #if __CHECKER__, so it doesn't matter what we
> > return as this code will never run.
> 
> It does matter slightly, as Sparse does some (very limited) value-based
> analyses. Let's future-proof it.
> 
> > That said, if sparse supports the GNU syntax of ?: then I have no
> > objection to doing that.
> 
> Sparse does support that syntax.

Great, I'll fix that and resubmit.

While I've got you, I've been looking at some other sparse warnings from
this file.  There are several caused by sparse being unable to handle
the following construct:

	if (foo)
		x = NULL;
	else {
		x = bar;
		__acquire(bar);
	}
	if (!x)
		return -ENOMEM;

Writing it as:

	if (foo)
		return -ENOMEM;
	else {
		x = bar;
		__acquire(bar);
	}

works just fine.  ie this removes the warning:

@@ -1070,9 +1070,9 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct
 mm_struct *src_mm,
 again:
        init_rss_vec(rss);
 
-       dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl);
-       if (!dst_pte)
+       if (pte_alloc(dst_mm, dst_pmd, addr))
                return -ENOMEM;
+       dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, addr, &dst_ptl);
        src_pte = pte_offset_map(src_pmd, addr);
        src_ptl = pte_lockptr(src_mm, src_pmd);
        spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);

Is there any chance sparse's dataflow analysis will be improved in the
near future?