Re: [PATCH] lockdep: Account for lockdep hardirq context in irq_forced_thread_fn under PREEMPT_RT

[Waiman Long <llong@redhat.com>]

On 10/6/25 2:34 PM, Guangbo Cui wrote:
> In PREEMPT_RT, IRQs are forced to run in threaded. However, lockdep did not correctly
> account for this case, causing false-positive warnings about hardirq context violations
> when analyzing lock acquisition in such threaded IRQs (see function `task_wait_context`).
>
> This patch updates `irq_forced_thread_fn` to explicitly call `lockdep_hardirq_enter()`
> and `lockdep_hardirq_exit()` when PREEMPT_RT is enabled, ensuring lockdep correctly
> tracks the hardirq context even when the IRQ is executed in a forced thread.
>
> This was discovered while testing PCIe AER error injection on an arm64 QEMU virtual machine:
>
> ```
>    qemu-system-aarch64 \
>        -nographic \
>        -machine virt,highmem=off,gic-version=3 \
>        -cpu cortex-a72 \
>        -kernel arch/arm64/boot/Image \
>        -initrd initramfs.cpio.gz \
>        -append "console=ttyAMA0 root=/dev/ram rdinit=/linuxrc earlyprintk nokaslr" \
>        -m 2G \
>        -smp 1 \
>        -netdev user,id=net0,hostfwd=tcp::2223-:22 \
>        -device virtio-net-pci,netdev=net0 \
>        -device pcie-root-port,id=rp0,chassis=1,slot=0x0 \
>        -device pci-testdev -s -S
> ```
>
> Injecting a correctable PCIe error via /dev/aer_inject caused a BUG
> report with "Invalid wait context" in the irq/PCIe thread.
>
> ```
> ~ # export HEX="00020000000000000100000000000000000000000000000000000000"
> ~ # echo -n "$HEX" | xxd -r -p | tee /dev/aer_inject >/dev/null
> [ 1850.947170] pcieport 0000:00:02.0: aer_inject: Injecting errors 00000001/00000000 into device 0000:00:02.0
> [ 1850.949951]
> [ 1850.950479] =============================
> [ 1850.950780] [ BUG: Invalid wait context ]
> [ 1850.951152] 6.17.0-11316-g7a405dbb0f03-dirty #7 Not tainted
> [ 1850.951457] -----------------------------
> [ 1850.951680] irq/16-PCIe PME/56 is trying to lock:
> [ 1850.952004] ffff800082865238 (inject_lock){+.+.}-{3:3}, at: aer_inj_read_config+0x38/0x1dc
> [ 1850.952731] other info that might help us debug this:
> [ 1850.952997] context-{5:5}
> [ 1850.953192] 5 locks held by irq/16-PCIe PME/56:
> [ 1850.953415]  #0: ffff800082647390 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x30/0x268
> [ 1850.953931]  #1: ffff8000826c6b38 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire+0x4/0x48
> [ 1850.954453]  #2: ffff000004bb6c58 (&data->lock){+...}-{3:3}, at: pcie_pme_irq+0x34/0xc4
> [ 1850.954949]  #3: ffff8000826c6b38 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire+0x4/0x48
> [ 1850.955420]  #4: ffff800082863d10 (pci_lock){....}-{2:2}, at: pci_bus_read_config_dword+0x5c/0xd8

data->lock is a rt_spin_lock and pci_lock is a raw_spinlock_t with irq 
disabled. So the data->lock => pci_lock sequence is OK. However, 
inject_lock is a rt_spin_lock again. So you can't acquire it with a 
raw_spinlock held and interrupt disabled. It is something that needs to 
be fixed not worked around as if it is OK. It is not a false positive.

Cheers,
Longman