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Date:   Mon, 6 Apr 2020 17:58:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Michel Lespinasse <walken@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Liam Howlett <Liam.Howlett@oracle.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        David Rientjes <rientjes@google.com>,
        Hugh Dickins <hughd@google.com>, Ying Han <yinghan@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Markus Elfring <Markus.Elfring@web.de>
Subject: Re: [PATCH v3 06/10] mmap locking API: convert nested write lock
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On Mon, Apr 06, 2020 at 08:35:03AM -0700, Michel Lespinasse wrote:
> On Wed, Apr 1, 2020 at 6:42 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Mar 27, 2020 at 03:50:58PM -0700, Michel Lespinasse wrote:
> >
> > > @@ -26,6 +31,12 @@ static inline void mmap_write_unlock(struct mm_struct *mm)
> > >       up_write(&mm->mmap_sem);
> > >  }
> > >
> > > +/* Pairs with mmap_write_lock_nested() */
> > > +static inline void mmap_write_unlock_nested(struct mm_struct *mm)
> > > +{
> > > +     up_write(&mm->mmap_sem);
> > > +}
> > > +
> > >  static inline void mmap_downgrade_write_lock(struct mm_struct *mm)
> > >  {
> > >       downgrade_write(&mm->mmap_sem);
> >
> > Why does unlock_nested() make sense ?
> 
> I thought it would make things more explicit to match the nested lock
> with the corresponding unlock site; however this information is not
> used at the moment (i.e. the nested unlock is implemented identically
> to the regular unlock).
> 
> Having the matching sites explicitly identified may help when
> implementing lock instrumentation, or when changing the lock type
> (another patchset I am working on needs to pass an explicit lock range
> to the nested lock and unlock sites).
> 
> I'll admit this is not a super strong argument, and can be deferred to
> when an actual need shows up in the future.

The thing is, lock-acquisition order matters _a_lot_, lock-release order
is irrelevant. ISTR there was a thread about this some 14 years ago, but
please don't ask me to go find it :/