On 2019/10/24 4:46, Bjorn Helgaas wrote: > [+cc Thomas, Rafael, beginning of thread at > https://lore.kernel.org/r/79827f2f-9b43-4411-1376-b9063b67aee3@xxxxxxxxxx] > > On Wed, Oct 23, 2019 at 09:38:51AM -0700, Matthew Wilcox wrote: >> On Wed, Oct 23, 2019 at 10:15:40AM -0500, Bjorn Helgaas wrote: >>> I don't like being one of a handful of callers of __add_wait_queue(), >>> so I like that solution from that point of view. >>> >>> The 7ea7e98fd8d0 ("PCI: Block on access to temporarily unavailable pci >>> device") commit log suggests that using __add_wait_queue() is a >>> significant optimization, but I don't know how important that is in >>> practical terms. Config accesses are never a performance path anyway, >>> so I'd be inclined to use add_wait_queue() unless somebody complains. >> >> Wow, this has got pretty messy in the umpteen years since I last looked >> at it. >> >> Some problems I see: >> >> 1. Commit df65c1bcd9b7b639177a5a15da1b8dc3bee4f5fa (tglx) says: >> >> x86/PCI: Select CONFIG_PCI_LOCKLESS_CONFIG >> >> All x86 PCI configuration space accessors have either their own >> serialization or can operate completely lockless (ECAM). >> >> Disable the global lock in the generic PCI configuration space accessors. >> >> The concept behind this patch is broken. We still need to lock out >> config space accesses when devices are undergoing D-state transitions. >> I would suggest that for the contention case that tglx is concerned about, >> we should have a pci_bus_read_config_unlocked_##size set of functions >> which can be used for devices we know never go into D states. > > Host bridges that can't do config accesses atomically, e.g., they have > something like the 0xcf8/0xcfc addr/data ports, need serialization. > CONFIG_PCI_LOCKLESS_CONFIG removes the use of pci_lock for that, and I > think that part makes sense regardless of whether devices can enter D > states. > > We *should* prevent config accesses during D-state transitions (per > PCIe r5.0, sec 5.9), but I don't think pci_lock ever did that. > pci_raw_set_power_state() contains delays, but that only prevents > accesses from the caller, not from other threads or from userspace. > I suppose we should also prevent accesses by other threads during > transitions done by ACPI, e.g., _PS0, _PS1, _PS2, _PS3. AFAICT we > don't do any of that. > > It looks like pci_lock currently: > > - Serializes all kernel config accesses system-wide in > pci_bus_read_config_##size() (unless CONFIG_PCI_LOCKLESS_CONFIG=y). > > - Serializes all userspace config accesses system-wide in > pci_user_read_config_##size() (this seems unnecessary when > CONFIG_PCI_LOCKLESS_CONFIG=y). > > - Serializes userspace config accesses with resets of the device via > the dev->block_cfg_access bit and waitqueue mechanism. > > - Serializes kernel and userspace config accesses with bus->ops > changes in pci_bus_set_ops() (except that we don't serialize > kernel config accesses if CONFIG_PCI_LOCKLESS_CONFIG=y, which is > probably a problem). But pci_bus_set_ops() is hardly used and I'm > not sure it's worth keeping it. > >> 2. Commit a2e27787f893621c5a6b865acf6b7766f8671328 (jan kiszka) >> exports pci_lock. I think this is a mistake; at best there should be >> accessors for the pci_lock. But I don't understand why it needs to >> exclude PCI config space changes throughout pci_check_and_set_intx_mask(). >> Why can it not do: >> >> - bus->ops->read(bus, dev->devfn, PCI_COMMAND, 4, &cmd_status_dword); >> + pci_read_config_dword(dev, PCI_COMMAND, &cmd_status_dword); >> >> 3. I don't understand why 511dd98ce8cf6dc4f8f2cb32a8af31ce9f4ba4a1 >> changed pci_lock to be a raw spinlock. The patch description >> essentially says "We need it for RT" which isn't terribly helpful. >> >> 4. Finally, getting back to the original problem report here, I wouldn't >> write this code this way today. There's no reason not to use the >> regular add_wait_queue etc. BUT! Why are we using this custom locking >> mechanism? It pretty much screams to me of an rwsem (reads/writes >> of config space take it for read; changes to config space accesses >> (disabling and changing of accessor methods) take it for write. > > So maybe the immediate thing is to just convert to add_wait_queue()? Hmmm... May I push a patch? :) > > There's a lot we could clean up here, but I think it would take a fair > bit of untangling before we actually solve this panic. > > Bjorn > > . > -- Thanks, Xiang
