linux-kernel - Re: [PATCH v5.5 09/10] mmap locking API: add mmap_assert_locked() and mmap_assert_write

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Message-ID: <20200519130614.GC189720@google.com>
Date:   Tue, 19 May 2020 06:06:14 -0700
From:   Michel Lespinasse <walken@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Liam Howlett <Liam.Howlett@...cle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH v5.5 09/10] mmap locking API: add mmap_assert_locked()
 and mmap_assert_write_locked()

On Mon, May 18, 2020 at 01:01:33PM +0200, Vlastimil Babka wrote:
> On 4/24/20 3:38 AM, Michel Lespinasse wrote:
> > +static inline void mmap_assert_locked(struct mm_struct *mm)
> > +{
> > +	VM_BUG_ON_MM(!lockdep_is_held_type(&mm->mmap_sem, -1), mm);
> > +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
> > +}
> > +
> > +static inline void mmap_assert_write_locked(struct mm_struct *mm)
> > +{
> > +	VM_BUG_ON_MM(!lockdep_is_held_type(&mm->mmap_sem, 0), mm);
> > +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
> > +}
> 
> I would remove VM_BUG_ON_MM() from the lockdep part. If kernel has lockdep
> enabled, it's already in heavy debugging mode enough so let's just use it and
> not depend on DEBUG_VM. Many sites you convert don't require DEBUG_VM for the
> lockdep checks.
> 
> With that you can also use the standard lockdep_assert_held() and
> lockdep_assert_held_write() wrappers.
> 
> If user has both lockdep and DEBUG_VM enabled, should we run both variants?
> Perhaps lockdep is enough as it's more comprehensive? Your initial v5 version
> was doing that.

Thanks, changed these to lockdep_assert_held() /
lockdep_assert_held_write() as suggested. This misses dumping out the
mm, but I think there is only limited value in that. I did keep the
rwsem_is_locked fallback as people had commented earlier about
getting assertions in the non-lockdep case. If both are enabled...
then we'll get somewhat redundant assertions, but I think that is fine
(better to keep the code simple than try to work around that).

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.