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[156.34.55.100]) by smtp.gmail.com with ESMTPSA id r40sm2076805qtr.57.2019.06.07.12.37.22 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 07 Jun 2019 12:37:22 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hZKfy-0005KN-CF; Fri, 07 Jun 2019 16:37:22 -0300 Date: Fri, 7 Jun 2019 16:37:22 -0300 From: Jason Gunthorpe To: Souptick Joarder Cc: linux-rdma@vger.kernel.org, Linux-MM , Jerome Glisse , Ralph Campbell , John Hubbard Subject: Re: [RFC PATCH 09/11] mm/hmm: Remove racy protection against double-unregistration Message-ID: <20190607193722.GS14802@ziepe.ca> References: <20190523153436.19102-1-jgg@ziepe.ca> <20190523153436.19102-10-jgg@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Jun 08, 2019 at 01:08:37AM +0530, Souptick Joarder wrote: > On Thu, May 23, 2019 at 9:05 PM Jason Gunthorpe wrote: > > > > From: Jason Gunthorpe > > > > No other register/unregister kernel API attempts to provide this kind of > > protection as it is inherently racy, so just drop it. > > > > Callers should provide their own protection, it appears nouveau already > > does, but just in case drop a debugging POISON. > > > > Signed-off-by: Jason Gunthorpe > > mm/hmm.c | 9 ++------- > > 1 file changed, 2 insertions(+), 7 deletions(-) > > > > diff --git a/mm/hmm.c b/mm/hmm.c > > index 46872306f922bb..6c3b7398672c29 100644 > > +++ b/mm/hmm.c > > @@ -286,18 +286,13 @@ EXPORT_SYMBOL(hmm_mirror_register); > > */ > > void hmm_mirror_unregister(struct hmm_mirror *mirror) > > { > > - struct hmm *hmm = READ_ONCE(mirror->hmm); > > - > > - if (hmm == NULL) > > - return; > > + struct hmm *hmm = mirror->hmm; > > How about remove struct hmm *hmm and replace the code like below - > > down_write(&mirror->hmm->mirrors_sem); > list_del_init(&mirror->list); > up_write(&mirror->hmm->mirrors_sem); > hmm_put(hmm); > memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm)); > > Similar to hmm_mirror_register(). I think we get there in patch 10, right? When the series is all done the function looks like this: void hmm_mirror_unregister(struct hmm_mirror *mirror) { struct hmm *hmm = mirror->hmm; down_write(&hmm->mirrors_sem); list_del(&mirror->list); up_write(&hmm->mirrors_sem); hmm_put(hmm); memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm)); } I think this mostly matches what you wrote above, or do you think we should s/hmm/mirror->hmm/ anyhow? I think Ralph just added that :) Regards, Jason