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Tsirkin" To: Greg KH Cc: Yao Hongbo , alikernel-developer@linux.alibaba.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] uio/uio_pci_generic: Introduce refcnt on open/release Message-ID: <20220413044246-mutt-send-email-mst@kernel.org> References: <1649833302-27299-1-git-send-email-yaohongbo@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 13, 2022 at 09:33:17AM +0200, Greg KH wrote: > On Wed, Apr 13, 2022 at 03:01:42PM +0800, Yao Hongbo wrote: > > If two userspace programs both open the PCI UIO fd, when one > > of the program exits uncleanly, the other will cause IO hang > > due to bus-mastering disabled. > > > > It's a common usage for spdk/dpdk to use UIO. So, introduce refcnt > > to avoid such problems. > > Why do you have multiple userspace programs opening the same device? > Shouldn't they coordinate? Or to restate, I think the question is, why not open the device once and pass the FD around? > > > > Fixes: 865a11f987ab("uio/uio_pci_generic: Disable bus-mastering on release") space missing before ( here . > > Reported-by: Xiu Yang > > Signed-off-by: Yao Hongbo > > --- > > Changes for v2: > > Use refcount_t instead of atomic_t to catch overflow/underflows. > > --- > > drivers/uio/uio_pci_generic.c | 16 +++++++++++++++- > > 1 file changed, 15 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c > > index e03f9b5..1a5e1fd 100644 > > --- a/drivers/uio/uio_pci_generic.c > > +++ b/drivers/uio/uio_pci_generic.c > > @@ -31,6 +31,7 @@ > > struct uio_pci_generic_dev { > > struct uio_info info; > > struct pci_dev *pdev; > > + refcount_t refcnt; > > }; > > > > static inline struct uio_pci_generic_dev * > > @@ -39,6 +40,14 @@ struct uio_pci_generic_dev { > > return container_of(info, struct uio_pci_generic_dev, info); > > } > > > > +static int open(struct uio_info *info, struct inode *inode) > > +{ > > + struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info); > > + > > + refcount_inc(&gdev->refcnt); > > + return 0; > > +} > > + > > static int release(struct uio_info *info, struct inode *inode) > > { > > struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info); > > @@ -51,7 +60,9 @@ static int release(struct uio_info *info, struct inode *inode) > >  * Note that there's a non-zero chance doing this will wedge the device > >  * at least until reset. > > */ > > - pci_clear_master(gdev->pdev); > > + if (refcount_dec_and_test(&gdev->refcnt)) > > + pci_clear_master(gdev->pdev); > > The goal here is to flush things when userspace closes the device, as > the comment says. So don't you want that to happen for when userspace > closes the file handle no matter who opened it? > > As this is a functional change, how is userspace going to "know" this > functionality is now changed or not? > > And if userspace really wants to open this multiple times, then properly > switch the code to only create the device-specific structures when open > is called. Otherwise you are sharing structures here that are not > intended to be shared, shouldn't you have your own private one? > > this feels odd. > > thanks, > > greg k-h Sigh. Maybe it was a mistake to do 865a11f987ab to begin with. Anyone doing DMA with UIO is already on very thin ice. But oh well. -- MST