Re: [PATCH v2 1/1] powerpc/eeh: fix recursive pci_lock_rescan_remove locking in EEH event handling

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Dec 10, 2025 at 08:25:59AM -0600, Narayana Murty N wrote:
> The recent commit 1010b4c012b0 ("powerpc/eeh: Make EEH driver device
> hotplug safe") restructured the EEH driver to improve synchronization
> with the PCI hotplug layer.
> 
> However, it inadvertently moved pci_lock_rescan_remove() outside its
> intended scope in eeh_handle_normal_event(), leading to broken PCI
> error reporting and improper EEH event triggering. Specifically,
> eeh_handle_normal_event() acquired pci_lock_rescan_remove() before
> calling eeh_pe_bus_get(), but eeh_pe_bus_get() itself attempts to
> acquire the same lock internally, causing nested locking and disrupting
> normal EEH event handling paths.
> 
> This patch adds a boolean parameter do_lock to _eeh_pe_bus_get(),
> with two public wrappers:
>     eeh_pe_bus_get() with locking enabled.
>     eeh_pe_bus_get_nolock() that skips locking.
> 
> Callers that already hold pci_lock_rescan_remove() now use
> eeh_pe_bus_get_nolock() to avoid recursive lock acquisition.
> 
> Additionally, pci_lock_rescan_remove() calls are restored to the correct
> position—after eeh_pe_bus_get() and immediately before iterating affected
> PEs and devices. This ensures EEH-triggered PCI removes occur under proper
> bus rescan locking without recursive lock contention.
> 
> The eeh_pe_loc_get() function has been split into two functions:
>     eeh_pe_loc_get(struct eeh_pe *pe) which retrieves the loc for given PE.
>     eeh_pe_loc_get_bus(struct pci_bus *bus) which retrieves the location
>     code for given bus.
> 
> This resolves lockdep warnings such as:
> <snip>
> [   84.964298] [    T928] ============================================
> [   84.964304] [    T928] WARNING: possible recursive locking detected
> [   84.964311] [    T928] 6.18.0-rc3 #51 Not tainted
> [   84.964315] [    T928] --------------------------------------------
> [   84.964320] [    T928] eehd/928 is trying to acquire lock:
> [   84.964324] [    T928] c000000003b29d58 (pci_rescan_remove_lock){+.+.}-{3:3}, at: pci_lock_rescan_remove+0x28/0x40
> [   84.964342] [    T928]
>                        but task is already holding lock:
> [   84.964347] [    T928] c000000003b29d58 (pci_rescan_remove_lock){+.+.}-{3:3}, at: pci_lock_rescan_remove+0x28/0x40
> [   84.964357] [    T928]
>                        other info that might help us debug this:
> [   84.964363] [    T928]  Possible unsafe locking scenario:
> 
> [   84.964367] [    T928]        CPU0
> [   84.964370] [    T928]        ----
> [   84.964373] [    T928]   lock(pci_rescan_remove_lock);
> [   84.964378] [    T928]   lock(pci_rescan_remove_lock);
> [   84.964383] [    T928]
>                        *** DEADLOCK ***
> 
> [   84.964388] [    T928]  May be due to missing lock nesting notation
> 
> [   84.964393] [    T928] 1 lock held by eehd/928:
> [   84.964397] [    T928]  #0: c000000003b29d58 (pci_rescan_remove_lock){+.+.}-{3:3}, at: pci_lock_rescan_remove+0x28/0x40
> [   84.964408] [    T928]
>                        stack backtrace:
> [   84.964414] [    T928] CPU: 2 UID: 0 PID: 928 Comm: eehd Not tainted 6.18.0-rc3 #51 VOLUNTARY
> [   84.964417] [    T928] Hardware name: IBM,9080-HEX POWER10 (architected) 0x800200 0xf000006 of:IBM,FW1060.00 (NH1060_022) hv:phyp pSeries
> [   84.964419] [    T928] Call Trace:
> [   84.964420] [    T928] [c0000011a7157990] [c000000001705de4] dump_stack_lvl+0xc8/0x130 (unreliable)
> [   84.964424] [    T928] [c0000011a71579d0] [c0000000002f66e0] print_deadlock_bug+0x430/0x440
> [   84.964428] [    T928] [c0000011a7157a70] [c0000000002fd0c0] __lock_acquire+0x1530/0x2d80
> [   84.964431] [    T928] [c0000011a7157ba0] [c0000000002fea54] lock_acquire+0x144/0x410
> [   84.964433] [    T928] [c0000011a7157cb0] [c0000011a7157cb0] __mutex_lock+0xf4/0x1050
> [   84.964436] [    T928] [c0000011a7157e00] [c000000000de21d8] pci_lock_rescan_remove+0x28/0x40
> [   84.964439] [    T928] [c0000011a7157e20] [c00000000004ed98] eeh_pe_bus_get+0x48/0xc0
> [   84.964442] [    T928] [c0000011a7157e50] [c000000000050434] eeh_handle_normal_event+0x64/0xa60
> [   84.964446] [    T928] [c0000011a7157f30] [c000000000051de8] eeh_event_handler+0xf8/0x190
> [   84.964450] [    T928] [c0000011a7157f90] [c0000000002747ac] kthread+0x16c/0x180
> [   84.964453] [    T928] [c0000011a7157fe0] [c00000000000ded8] start_kernel_thread+0x14/0x18

I have no comment on the patch itself, but the timestamps above aren't
relevant and could be removed.  Maybe also the "T928" part.

Bjorn