From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Return-Path: Message-ID: <61e3dfcf831fab4443b86579f39d4f08c6200c41.camel@kernel.crashing.org> Subject: Re: [RFC PATCH 1/6] Revert "PCI: Fix is_added/is_busmaster race condition" From: Benjamin Herrenschmidt To: Lukas Wunner Cc: Bjorn Helgaas , linux-pci@vger.kernel.org, Hari Vyas , Ray Jui , Srinath Mannam , Guenter Roeck , Jens Axboe , Konstantin Khlebnikov , Marta Rybczynska , Pierre-Yves Kerbrat , linux-kernel@vger.kernel.org Date: Mon, 20 Aug 2018 21:12:07 +1000 In-Reply-To: <20180820071703.hnsyhb5pz4svb7jg@wunner.de> References: <20180817044902.31420-1-benh@kernel.crashing.org> <20180817044902.31420-2-benh@kernel.crashing.org> <20180817154431.GC128050@bhelgaas-glaptop.roam.corp.google.com> <06c1233b71dea08b1fc32334acefc48c32c46557.camel@kernel.crashing.org> <20180819022440.GG128050@bhelgaas-glaptop.roam.corp.google.com> <4d777ed8c17b479e59b16cc0b4e9a3f6537f9884.camel@kernel.crashing.org> <20180820071703.hnsyhb5pz4svb7jg@wunner.de> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 List-ID: On Mon, 2018-08-20 at 09:17 +0200, Lukas Wunner wrote: > On Mon, Aug 20, 2018 at 12:10:59PM +1000, Benjamin Herrenschmidt wrote: > > I chose to create a new mutex which we should be able to address other > > similar races if we find them. The other solutions that I dismissed > > were: > > > > - Using the device_lock. A explained previously, this is tricky, I > > prefer not using this for anything other than locking against > > concurrent add/remove. The main issue is that drivers will be sometimes > > called in context where that's already held, so we can't take it inside > > pci_enable_device() and I'd rather not add new constraints such as > > "pci_enable_device() must be only called from probe() unless you also > > take the device lock". It would be tricky to audit everybody... > > > > - Using a global mutex. We could move the bridge lock from AER to core > > code for example, and use that. But it doesn't buy us much, and > > slightly redecuces parallelism. It also makes it a little bit more > > messy to walk up the bridge chain, we'd have to do a > > pci_enable_device_unlocked or something, messy. > > +1 from my side for adding a struct mutex to struct pci_dev to protect > state changes. Ok thanks. This is what my patch proposes. We can use it later to protect more things if we wish to do so. > The device_lock() primarily protects binding / unbinding of the device > and pci_dev state may have to be changed while binding / unbinding. Yup, precisely. > A global lock invites deadlocks if multiple devices are added / removed > concurrently where one is a parent of the other. (Think hot-removal of > multiple devices on a Thunderbolt daisy-chain.) Yes. > As said I'd also welcome folding PCI_DEV_DISCONNECTED into enum > pci_channel_state, either as an additional state or by using > pci_channel_io_perm_failure. Ok. I have that in my tentative series but I think for robustness, I should make the error_state field atomically updated in order to ensure that no transition out of "disconnected" can happen while racing with concurrent error_state updates at interrupt time (at least with EEH, it can be updated from any read{b,w,l,q}). I'll do a bit more work on the patches this week as time permits and send a non-RFC series. Cheers, Ben. > Thanks, > > Lukas