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SubjectRe: [RFC PATCH] nvme: avoid race-conditions when enabling devices
[+cc Srinath]

On Wed, Mar 21, 2018 at 05:10:56PM +0100, Marta Rybczynska wrote:
> > On Wed, Mar 21, 2018 at 11:48:09PM +0800, Ming Lei wrote:
> >> On Wed, Mar 21, 2018 at 01:10:31PM +0100, Marta Rybczynska wrote:
> >> > > On Wed, Mar 21, 2018 at 12:00:49PM +0100, Marta Rybczynska wrote:
> >> > >> NVMe driver uses threads for the work at device reset, including enabling
> >> > >> the PCIe device. When multiple NVMe devices are initialized, their reset
> >> > >> works may be scheduled in parallel. Then pci_enable_device_mem can be
> >> > >> called in parallel on multiple cores.
> >> > >>
> >> > >> This causes a loop of enabling of all upstream bridges in
> >> > >> pci_enable_bridge(). pci_enable_bridge() causes multiple operations
> >> > >> including __pci_set_master and architecture-specific functions that
> >> > >> call ones like and pci_enable_resources(). Both __pci_set_master()
> >> > >> and pci_enable_resources() read PCI_COMMAND field in the PCIe space
> >> > >> and change it. This is done as read/modify/write.
> >> > >>
> >> > >> Imagine that the PCIe tree looks like:
> >> > >> A - B - switch - C - D
> >> > >> \- E - F
> >> > >>
> >> > >> D and F are two NVMe disks and all devices from B are not enabled and bus
> >> > >> mastering is not set. If their reset work are scheduled in parallel the two
> >> > >> modifications of PCI_COMMAND may happen in parallel without locking and the
> >> > >> system may end up with the part of PCIe tree not enabled.
> >> > >
> >> > > Then looks serialized reset should be used, and I did see the commit
> >> > > 79c48ccf2fe ("nvme-pci: serialize pci resets") fixes issue of 'failed
> >> > > to mark controller state' in reset stress test.
> >> > >
> >> > > But that commit only covers case of PCI reset from sysfs attribute, and
> >> > > maybe other cases need to be dealt with in similar way too.
> >> > >
> >> >
> >> > It seems to me that the serialized reset works for multiple resets of the
> >> > same device, doesn't it? Our problem is linked to resets of different devices
> >> > that share the same PCIe tree.
> >>
> >> Given reset shouldn't be a frequent action, it might be fine to serialize all
> >> reset from different devices.
> >
> > The driver was much simpler when we had serialized resets in line with
> > probe, but that had a bigger problems with certain init systems when
> > you put enough nvme devices in your server, making them unbootable.
> >
> > Would it be okay to serialize just the pci_enable_device across all
> > other tasks messing with the PCI topology?
> >
> > ---
> > diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> > index cef5ce851a92..e0a2f6c0f1cf 100644
> > --- a/drivers/nvme/host/pci.c
> > +++ b/drivers/nvme/host/pci.c
> > @@ -2094,8 +2094,11 @@ static int nvme_pci_enable(struct nvme_dev *dev)
> > int result = -ENOMEM;
> > struct pci_dev *pdev = to_pci_dev(dev->dev);
> >
> > - if (pci_enable_device_mem(pdev))
> > - return result;
> > + pci_lock_rescan_remove();
> > + result = pci_enable_device_mem(pdev);
> > + pci_unlock_rescan_remove();
> > + if (result)
> > + return -ENODEV;
> >
> > pci_set_master(pdev);
>
> The problem may happen also with other device doing its probe and
> nvme running its workqueue (and we probably have seen it in practice
> too). We were thinking about a lock in the pci generic code too,
> that's why I've put the linux-pci@ list in copy.

Yes, this is a generic problem in the PCI core. We've tried to fix it
in the past but haven't figured it out yet.

See 40f11adc7cd9 ("PCI: Avoid race while enabling upstream bridges")
and 0f50a49e3008 ("Revert "PCI: Avoid race while enabling upstream
bridges"").

It's not trivial, but if you figure out a good way to fix this, I'd be
thrilled.

Bjorn

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