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Date:   Wed, 21 Mar 2018 10:02:39 -0600
From:   Keith Busch <keith.busch@intel.com>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Marta Rybczynska <mrybczyn@kalray.eu>, axboe@fb.com, hch@lst.de,
        sagi@grimberg.me, linux-nvme@lists.infradead.org,
        linux-kernel@vger.kernel.org, bhelgaas@google.com,
        linux-pci@vger.kernel.org, Pierre-Yves Kerbrat <pkerbrat@kalray.eu>
Subject: Re: [RFC PATCH] nvme: avoid race-conditions when enabling devices
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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);

--