On Fri, Apr 26, 2019 at 10:11:54AM +0200, CREGUT Pierre IMT/OLN wrote: > I also initially thought that kobject_uevent generated the netlink event > but this is not the case. This is generated by the specific driver in use. > For the Intel i40e driver, this is the call to i40e_do_reset_safe in > i40e_pci_sriov_configure that sends the event. > It is followed by i40e_pci_sriov_enable that calls i40e_alloc_vfs that > finally calls the generic pci_enable_sriov function. I don't know anything about netlink. The script from the bugzilla (https://bugzilla.kernel.org/show_bug.cgi?id=202991) looks like it runs ip monitor dev enp9s0f2 What are the actual netlink events you see? Are they related to a device being removed? When we change num_VFs, I think we have to disable any existing VFs before enabling the new num_VFs, so if you trigger on a netlink "remove" event, I wouldn't be surprised that reading sriov_numvfs would give a zero until the new VFs are enabled. > So the proposed patch works well for the i40e driver (x710 cards) because > the update to num_VFs is fast enough to be committed before the event is > received. It may not work with other cards. The same is true for the zero > value and there is no guarantee for other cards. > > The clean solution would be to lock the device in sriov_numvfs_show. > I guess that there are good reasons why locks have been avoided > in sysfs getter functions so let us explore other approaches. > > We can either return a "not settled" value (-1) or (probably better) > do not return a value but an error (-EAGAIN returned by the show > function). > > To distinguish this "not settled" situation we can either: > * overload the meaning of num_VFs (eg make it negative) > but it is an unsigned short. > * add a bool to pci_sriov struct (rather simple but modifies a well > established structure). > * use the fact that not_settled => device is locked and use > mutex_is_locked as an over approximation. > > The later is not perfect but requires minimal changes to > sriov_numvfs_show: > > if (mutex_is_locked(&dev->mutex)) > return -EAGAIN; I thought this was a good idea, but - It does break the device_lock() encapsulation a little bit: sriov_numvfs_store() uses device_lock(), which happens to be implemented as "mutex_lock(&dev->mutex)", but we really shouldn't rely on that implementation, and - The netlink events are being generated via the NIC driver, and I'm a little hesitant about changing the PCI core to deal with timing issues "over there". > In all cases, the device could be locked or the boolean set just > after the test. But I don't think there is a case where causality > would be violated.Thank you in advance for your recommendations. I will > update the patch according to your instructions. > > Le 06/04/2019 à 00:33, Bjorn Helgaas a écrit : > > On Fri, Mar 29, 2019 at 09:00:58AM +0100, Pierre Crégut wrote: > > > Ensure that iov->num_VFs is set before a netlink message is sent > > > when the number of VFs is changed. Only the path for num_VFs > 0 > > > is affected. The path for num_VFs = 0 is already correct. > > > > > > Monitoring programs can relie on netlink messages to track interface > > > change and query their state in /sys. But when sriov_numvfs is set to a > > > positive value, the netlink message is sent before the value is available > > > in sysfs. The value read after the message is received is always zero. > > Thanks, Pierre! Can you clue me in on where exactly the connection > > from sriov_enable() to netlink is? > > > > I see one side of the race is with sriov_numvfs_show(), but I don't > > know where the netlink message is sent. Is that connected with the > > kobject_uevent(KOBJ_CHANGE)? > > > > One thing this would help with is figuring out exactly how *much* > > earlier we need to set iov->num_VFs. It looks like the current patch > > sets it before we actually enable the VFs, so a user could read > > /sys/.../sriov_numvfs and get the wrong value. Of course, that's > > unavoidable; the question is whether it's OK to get the new value > > *before* it actually takes effect, or whether we want to return a > > stale value until after it takes effect. > > > > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=202991 > > > Signed-off-by: Pierre Crégut <pierre.cregut@xxxxxxxxxx> > > > --- > > > note: the behaviour can be tested with the following shell script also > > > available on the bugzilla (d being the phy device name): > > > > > > ip monitor dev $d | grep --line-buffered "^[0-9]*:" | \ > > > while read line; do cat /sys/class/net/$d/device/sriov_numvfs; done > > > > > > drivers/pci/iov.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c > > > index 3aa115ed3a65..a9655c10e87f 100644 > > > --- a/drivers/pci/iov.c > > > +++ b/drivers/pci/iov.c > > > @@ -351,6 +351,7 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn) > > > goto err_pcibios; > > > } > > > + iov->num_VFs = nr_virtfn; > > > pci_iov_set_numvfs(dev, nr_virtfn); > > > iov->ctrl |= PCI_SRIOV_CTRL_VFE | PCI_SRIOV_CTRL_MSE; > > > pci_cfg_access_lock(dev); > > > @@ -363,7 +364,6 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn) > > > goto err_pcibios; > > > kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE); > > > - iov->num_VFs = nr_virtfn; > > > return 0; > > > @@ -379,6 +379,7 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn) > > > if (iov->link != dev->devfn) > > > sysfs_remove_link(&dev->dev.kobj, "dep_link"); > > > + iov->num_VFs = 0; > > > pci_iov_set_numvfs(dev, 0); > > > return rc; > > > } > > > -- > > > 2.17.1 > > >
