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[156.34.55.100]) by smtp.gmail.com with ESMTPSA id m38sm12868061qta.43.2019.08.13.04.48.42 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 13 Aug 2019 04:48:42 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hxVIA-0007l2-Ec; Tue, 13 Aug 2019 08:48:42 -0300 Date: Tue, 13 Aug 2019 08:48:42 -0300 From: Jason Gunthorpe To: Ira Weiny Cc: Andrew Morton , Dan Williams , Matthew Wilcox , Jan Kara , Theodore Ts'o , John Hubbard , Michal Hocko , Dave Chinner , linux-xfs@vger.kernel.org, linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v2 16/19] RDMA/uverbs: Add back pointer to system file object Message-ID: <20190813114842.GB29508@ziepe.ca> References: <20190809225833.6657-1-ira.weiny@intel.com> <20190809225833.6657-17-ira.weiny@intel.com> <20190812130039.GD24457@ziepe.ca> <20190812172826.GA19746@iweiny-DESK2.sc.intel.com> <20190812175615.GI24457@ziepe.ca> <20190812211537.GE20634@iweiny-DESK2.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190812211537.GE20634@iweiny-DESK2.sc.intel.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-rdma-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org On Mon, Aug 12, 2019 at 02:15:37PM -0700, Ira Weiny wrote: > On Mon, Aug 12, 2019 at 02:56:15PM -0300, Jason Gunthorpe wrote: > > On Mon, Aug 12, 2019 at 10:28:27AM -0700, Ira Weiny wrote: > > > On Mon, Aug 12, 2019 at 10:00:40AM -0300, Jason Gunthorpe wrote: > > > > On Fri, Aug 09, 2019 at 03:58:30PM -0700, ira.weiny@intel.com wrote: > > > > > From: Ira Weiny > > > > > > > > > > In order for MRs to be tracked against the open verbs context the ufile > > > > > needs to have a pointer to hand to the GUP code. > > > > > > > > > > No references need to be taken as this should be valid for the lifetime > > > > > of the context. > > > > > > > > > > Signed-off-by: Ira Weiny > > > > > drivers/infiniband/core/uverbs.h | 1 + > > > > > drivers/infiniband/core/uverbs_main.c | 1 + > > > > > 2 files changed, 2 insertions(+) > > > > > > > > > > diff --git a/drivers/infiniband/core/uverbs.h b/drivers/infiniband/core/uverbs.h > > > > > index 1e5aeb39f774..e802ba8c67d6 100644 > > > > > +++ b/drivers/infiniband/core/uverbs.h > > > > > @@ -163,6 +163,7 @@ struct ib_uverbs_file { > > > > > struct page *disassociate_page; > > > > > > > > > > struct xarray idr; > > > > > + struct file *sys_file; /* backpointer to system file object */ > > > > > }; > > > > > > > > The 'struct file' has a lifetime strictly shorter than the > > > > ib_uverbs_file, which is kref'd on its own lifetime. Having a back > > > > pointer like this is confouding as it will be invalid for some of the > > > > lifetime of the struct. > > > > > > Ah... ok. I really thought it was the other way around. > > > > > > __fput() should not call ib_uverbs_close() until the last reference on struct > > > file is released... What holds references to struct ib_uverbs_file past that? > > > > Child fds hold onto the internal ib_uverbs_file until they are closed > > The FDs hold the struct file, don't they? Only dups, there are other 'child' FDs we can create > > Now this has unlocked updates to that data.. you'd need some lock and > > get not zero pattern > > You can't call "get" here because I'm 99% sure we only get here when struct > file has no references left... Nope, like I said the other FDs hold the uverbs_file independent of the struct file it is related too. This is why having a back pointer like this is so ugly, it creates a reference counting cycle Jason