Gentle ping on this RFC. Lukas, Bjorn, could you let me know whether the general direction is acceptable before I work on a non-RFC version? Any comments on the approach would help.
Thanks, Abhin On Sun, Sep 27, 2026 at 06:20:11PM +0000, Abhin Parekadan Jose wrote: > If a safe removal is already in progress when the device is surprise > removed, pciehp cannot report the disconnect [1]. The removal blocks > waiting on a device interrupt or status read, and pciehp's IRQ thread is > single-threaded and is itself executing that removal, so it never runs > again to report the device gone. The removal hangs indefinitely. > > pciehp_isr() does run while the IRQ thread is blocked, but pciehp_ist() > must ignore link and presence changes caused by Secondary Bus Reset or > DPC, and telling those apart takes seconds which cannot be spent in > hardirq. > > Patch 4 does not do that work in hardirq. pciehp_isr() only checks > whether the card is present or the link is active, and defers the rest > to a work item in process context. The work item does not need to know > why the link changed, only whether the card is gone: it waits for DPC > recovery or a Secondary Bus Reset in progress to complete, then checks > again under reset_lock, and if the card is absent and the link is down > marks the devices below disconnected, which schedules the driver's > disconnect work from patch 1. pciehp_ist() is unchanged and > remains the only consumer of the one-shot flags PCI_DPC_RECOVERED and > PCI_LINK_CHANGED, which tell it whether a link change can be ignored. > > Patches: > > 1/5 Michael's "PCI: Report surprise removal event" from his RFC v5, > which adds disconnect_work_enable and pdev->disconnect_work. > Changed to serialize disconnect_work_enable with a per-device > spinlock. > > 2/5 Add pci_hp_wait_link_change(), which awaits a Secondary Bus Reset > in progress without consuming PCI_LINK_CHANGED. > > 3/5 Add pci_dpc_wait_recovery(), which awaits DPC recovery without > consuming PCI_DPC_RECOVERED. > > 4/5 The pciehp change. Adds disconnect_work to struct controller, > scheduled from pciehp_isr() on PDC or DLLSC when neither Presence > Detect State nor Data Link Layer Link Active indicates a card. > > 5/5 A POC driver for the QEMU edu device that blocks in remove() > waiting for an interrupt, standing in for del_gendisk() stuck in > blk_mq_freeze_queue_wait(). Not for merge -- included so the > hang can be reproduced. > > Changes since RFC v3 [5]: > > - 5/5: Reinitialize the completion in remove() before requesting the > delayed interrupt, so that an earlier completion cannot let remove() > return while the interrupt is still pending. (Sashiko) > - 1/5 to 4/5: No changes. > > Changes since RFC v2 [4]: > > - 1/5: Protect disconnect_work_enable with a per-device spinlock, held > while testing it and scheduling the work, instead of lockless > accesses and barriers. pci_dev_set_disconnected() could otherwise > test the flag, get preempted, and queue the work while a newly bound > driver re-initializes it. With the lock, cancel_work_sync() is > sufficient again, so go back to it from disable_work_sync(). > (Sashiko) > - 2/5: Test PCI_LINK_CHANGING with test_bit_acquire(), so that the > caller's subsequent accesses are ordered after the end of the code > section even if wait_event() returns without sleeping. (Sashiko) > - 3/5, 4/5, 5/5: No changes. > > Changes since RFC v1 [2]: > > - 1/5: Use disable_work_sync() instead of cancel_work_sync() in > pci_clear_disconnect_work(), so that a racing schedule_work() cannot > queue the work after remove() has returned. (Sashiko) > - 2/5, 3/5: New. > - 4/5: Drop schedule_notification_work(); pci_dev_set_disconnected() > schedules the driver's disconnect work for all callers again. > (Michael) > - 4/5: Don't consume PCI_DPC_RECOVERED and PCI_LINK_CHANGED in the > work item. In v1 it ran the same spurious link change test as > pciehp_ist(), and whichever ran first took the flags, so pciehp_ist() > could tear down a device that was only reset. (Sashiko) > - 4/5: Check presence with pciehp_card_present_or_link_active() in > both pciehp_isr() and the work item, as pciehp_ist() does, so that a > port with Presence Detect State hardwired to zero is not mistaken > for an empty slot. > - 4/5: Return early from the work item if pciehp_ist() has already > taken the pending events, and treat a read error of the presence > check as "not present". (Sashiko) > - 4/5: In pciehp_isr(), check presence before dropping the runtime PM > reference on the port's parent. In the work item, take a runtime PM > reference and check presence under reset_lock, since a slot reset > may make Presence Detect State and Link Active flap. > - 5/5: Build only with CONFIG_EDU_SRPOC, which depends on PCI; don't > claim the interrupt when the device reads all ones; clear bus > mastering on teardown. (Sashiko) > > Testing > > Reproducing this needs QEMU changes, since neither device_del nor the > attention button produces a true surprise removal. A branch with both > is here [3]: > > - a delayed-IRQ register on the edu device (BAR0 0x30, write N ms) > - a pcie_surprise_del monitor command that drops the device and > generates PDC=1, DLLSC=1, PDS=0 > > Both tests use: > > ./qemu-system-aarch64 -machine virt,gic-version=3 -cpu cortex-a57 \ > -m 512 -smp 2 -kernel Image -initrd initramfs.cpio.gz \ > -device pcie-root-port,id=rp1,chassis=1,slot=1 \ > -device edu,bus=rp1,id=edu0 -append "console=ttyAMA0 rdinit=/init" \ > -nographic -monitor unix:/tmp/qemu-mon.sock,server,nowait > > and need a guest kernel with CONFIG_EDU_SRPOC=y. > > Test 1 (Hang in remove() on a user thread, then surprise removal): > > guest# echo 1 > /sys/bus/pci/devices/0000:01:00.0/remove & > host$ echo "pcie_surprise_del edu0" | socat - > unix-connect:/tmp/qemu-mon.sock > > This is the case patch 1 solves on its own: pciehp's IRQ thread is > free and handles the removal. > > Test 2 (Hang in remove() on pciehp's IRQ thread, then surprise removal): > > guest# echo 0 > /sys/bus/pci/slots/1/power & > host$ echo "pcie_surprise_del edu0" | socat - > unix-connect:/tmp/qemu-mon.sock > > Without patch 4 the safe removal does not return until the delayed > interrupt fires 600 s later. With it, > pciehp_isr() schedules ctrl->disconnect_work, which marks the device > disconnected; the POC driver's disconnect work completes the wait and > remove() proceeds. > > Open questions > > - Is this a viable approach? > > [1] https://lore.kernel.org/all/[email protected]/ > [2] https://lore.kernel.org/all/[email protected]/ > [3] https://gitlab.com/abhinkop/qemu/-/commits/suprise-removal > [4] https://lore.kernel.org/all/[email protected]/ > [5] https://lore.kernel.org/all/[email protected]/ > > Assisted-by: LLM > > Abhin Parekadan Jose (4): > PCI: pciehp: Add pci_hp_wait_link_change() > PCI/DPC: Add pci_dpc_wait_recovery() > PCI: pciehp: Report surprise removal from pciehp_isr() > misc: Add edu_srpoc surprise removal POC driver > > Michael S. Tsirkin (1): > PCI: Report surprise removal event > > drivers/misc/Kconfig | 11 ++ > drivers/misc/Makefile | 1 + > drivers/misc/edu_srpoc.c | 172 +++++++++++++++++++++++++ > drivers/pci/hotplug/pci_hotplug_core.c | 21 ++- > drivers/pci/hotplug/pciehp.h | 1 + > drivers/pci/hotplug/pciehp_hpc.c | 67 ++++++++++ > drivers/pci/pci.h | 10 ++ > drivers/pci/pcie/dpc.c | 54 ++++++-- > drivers/pci/probe.c | 1 + > include/linux/pci.h | 57 ++++++++ > 10 files changed, 381 insertions(+), 14 deletions(-) > create mode 100644 drivers/misc/edu_srpoc.c > > > base-commit: fd179f8a05be3ccae366b9b96e176b51fbe54aab > -- > 2.51.1 >
