From: Bart Van Assche <bvanassche@acm.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org,
Johannes Berg <johannes.berg@intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH RFC] kernel/locking, fs/direct-io: Introduce and use down_write_nolockdep()
Date: Fri, 26 Oct 2018 11:11:18 -0700 [thread overview]
Message-ID: <1540577478.66186.108.camel@acm.org> (raw)
In-Reply-To: <20181026174352.GT25444@bombadil.infradead.org>
On Fri, 2018-10-26 at 10:43 -0700, Matthew Wilcox wrote:
> On Fri, Oct 26, 2018 at 09:49:05AM -0700, Bart Van Assche wrote:
> > +++ b/include/linux/rwsem.h
> > @@ -41,6 +41,10 @@ struct rw_semaphore {
> > #endif
> > #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > struct lockdep_map dep_map;
> > + /*
> > + * Number of up_write() calls that must skip rwsem_release().
> > + */
> > + unsigned nolockdep;
>
> This reads a bit weird. By definition, only one writer is allowed
> at a time. And you can't call up_write() before down_write().
> So functionally, this is a bool, and the comment should at least
> ackowledge that, even if it remains implemented as an unsigned int.
>
> I'd suggest the implementation uses '= 1' and '= 0' rather than ++ and --.
Hi Matthew,
That sounds like a good idea to me.
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 1e79fac3186b..2a953d3b7431 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -81,6 +81,7 @@ static inline struct anon_vma *anon_vma_alloc(void)
> >
> > anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
> > if (anon_vma) {
> > + init_rwsem(&anon_vma->rwsem);
> > atomic_set(&anon_vma->refcount, 1);
> > anon_vma->degree = 1; /* Reference for first vma */
> > anon_vma->parent = anon_vma;
>
> Why is this needed? The anon_vma_ctor() already calls init_rwsem().
>
> (I suspect this is one of those ctors that isn't actually useful and
> should be inlined into anon_vma_alloc())
Without that call I noticed that the "nolockdep" variable was sometimes set
when down_write() got called. Does that mean that it can happen that an
anon_vma structure is freed without releasing anon_vma->rwsem?
Thanks,
Bart.
next prev parent reply other threads:[~2018-10-27 2:49 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-26 16:49 [PATCH RFC] kernel/locking, fs/direct-io: Introduce and use down_write_nolockdep() Bart Van Assche
2018-10-26 17:43 ` Matthew Wilcox
2018-10-26 18:11 ` Bart Van Assche [this message]
2018-10-26 18:38 ` Matthew Wilcox
2018-10-26 19:12 ` Bart Van Assche
2018-10-27 5:37 ` Dave Chinner
2018-10-28 17:58 ` Peter Zijlstra
2018-10-28 20:34 ` Christoph Hellwig
2018-10-28 20:45 ` Bart Van Assche
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