On 18.01.2023 19:21, Julien Grall wrote: > On 06/09/2022 11:32, Jan Beulich wrote: >> On 31.08.2022 16:10, Volodymyr Babchuk wrote: >>> Hello, >>> >>> This is yet another take to a PCI locking rework. This approach >>> was suggest by Jan Beulich who proposed to use a reference >>> counter to control lifetime of pci_dev objects. >>> >>> When I started added reference counting it quickly became clear >>> that this approach can provide more granular locking insted of >>> huge pcidevs_lock() which is used right now. I studied how this >>> lock used and what it protects. And found the following: >>> >>> 0. Comment in pci.h states the following: >>> >>> 153 /* >>> 154 * The pcidevs_lock protect alldevs_list, and the assignment for the >>> 155 * devices, it also sync the access to the msi capability that is not >>> 156 * interrupt handling related (the mask bit register). >>> 157 */ >>> >>> But in reality it does much more. Here is what I found: >>> >>> 1. Lifetime of pci_dev struct >>> >>> 2. Access to pseg->alldevs_list >>> >>> 3. Access to domain->pdev_list >>> >>> 4. Access to iommu->ats_list >>> >>> 5. Access to MSI capability >>> >>> 6. Some obsucure stuff in IOMMU drivers: there are places that >>> are guarded by pcidevs_lock() but it seems that nothing >>> PCI-related happens there. >> >> Right - the lock being held was (ab)used in IOMMU code in a number of >> places. This likely needs to change in the course of this re-work; >> patch titles don't suggest this is currently part of the series. >> >>> 7. Something that I probably overlooked >> >> And this is the main risk here. The huge scope of the original lock >> means that many things are serialized now but won't be anymore once >> the lock is gone. >> >> But yes - thanks for the work. To be honest I don't expect to be able >> to look at this series in detail until after the Xen Summit. And even >> then it may take a while ... > > I was wondering if this is still in your list to review?
Yes, it certainly is. But as before no predictions when I might get to it. Jan
