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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id r64sm6400897qkf.119.2020.10.09.07.32.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 09 Oct 2020 07:32:10 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kQtRJ-001zsY-G1; Fri, 09 Oct 2020 11:32:09 -0300 Date: Fri, 9 Oct 2020 11:32:09 -0300 From: Jason Gunthorpe To: Daniel Vetter Cc: DRI Development , LKML , KVM list , Linux MM , Linux ARM , linux-samsung-soc , "open list:DMA BUFFER SHARING FRAMEWORK" , linux-s390 , Daniel Vetter , Kees Cook , Dan Williams , Andrew Morton , John Hubbard , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Jan Kara , Arnd Bergmann , Greg Kroah-Hartman , David Hildenbrand , "Rafael J. Wysocki" Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework Message-ID: <20201009143209.GS5177@ziepe.ca> References: <20201009075934.3509076-1-daniel.vetter@ffwll.ch> <20201009075934.3509076-15-daniel.vetter@ffwll.ch> <20201009123109.GO5177@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote: > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe wrote: > > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote: > > > > > +struct address_space *iomem_get_mapping(void) > > > +{ > > > + return iomem_inode->i_mapping; > > > > This should pair an acquire with the release below > > > > > + /* > > > + * Publish /dev/mem initialized. > > > + * Pairs with smp_load_acquire() in revoke_iomem(). > > > + */ > > > + smp_store_release(&iomem_inode, inode); > > > > However, this seems abnormal, initcalls rarely do this kind of stuff > > with global data.. > > > > The kernel crashes if this fs_initcall is raced with > > iomem_get_mapping() due to the unconditional dereference, so I think > > it can be safely switched to a simple assignment. > > Ah yes I checked this all, but forgot to correctly annotate the > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem: > Add missing memory barriers for devmem_inode"). Oh yikes, so revoke_iomem can run concurrently during early boot, tricky. > The reasons for the annotations is that iomem requests can happen > fairly early, way before fs_initcalls happen. That means revoke_iomem > needs to check for that and bail out if we race - nothing bad can > happen since userspace isn't running at this point anyway. And > apparently it needs to be a full acquire fence since we don't just > write a value, but need a barrier for the struct stuff. Yes, if that is what is happening it release/acquire is needed. Jason