Re: [PATCH RFC v4 0/5] PCI: pciehp: Report surprise removal during safe removal

[Abhin Parekadan Jose <abhinjoses@gmail.com>]

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/aHlZE18kPuHuDtTT@wunner.de/
> [2] https://lore.kernel.org/all/20260905183905.997833-1-abhinjoses@gmail.com/
> [3] https://gitlab.com/abhinkop/qemu/-/commits/suprise-removal
> [4] https://lore.kernel.org/all/20260927165459.829900-1-abhinjoses@gmail.com/
> [5] https://lore.kernel.org/all/20260927175203.928270-1-abhinjoses@gmail.com/
> 
> 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
>