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

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Date:   Sat, 23 Dec 2017 05:06:21 -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>,
        linux-sparse@...r.kernel.org
Subject: Re: [PATCH 2/2] Introduce __cond_lock_err

On Sat, Dec 23, 2017 at 01:39:11AM -0800, Josh Triplett wrote:
> +linux-sparse

Ehh ... we've probably trimmed too much to give linux-sparse a good summary.

Here're the important lines from my patch:

+# define __cond_lock_err(x,c)  ((c) ? 1 : ({ __acquire(x); 0; }))

+       return __cond_lock_err(*ptlp, __follow_pte_pmd(mm, address, start, end,
+                                                   ptepp, pmdpp, ptlp));

This is supposed to be "If "c" is an error value, we don't have a lock,
otherwise we have a lock".  And to translate from linux-speak into
sparse-speak:

 # define __acquire(x)  __context__(x,1)

Josh & Ross pointed out (quite correctly) that code which does something like

if (foo())
	return;

will work with this, but code that does

if (foo() < 0)
	return;

will not because we're now returning 1 instead of -ENOMEM (for example).

So they made the very sensible suggestion that I change the definition
of __cond_lock to:

# define __cond_lock_err(x,c)  ((c) ?: ({ __acquire(x); 0; }))

Unfortunately, when I do that, the context imbalance warning returns.
As I said below, this is with sparse 0.5.1.

> On Fri, Dec 22, 2017 at 05:36:34AM -0800, Matthew Wilcox wrote:
> > On Fri, Dec 22, 2017 at 04:31:12AM -0800, Matthew Wilcox wrote:
> > > 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.
> > 
> > Except the context imbalance warning comes back if I do.  This is sparse
> > 0.5.1 (Debian's 0.5.1-2 package).
> 
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