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SubjectRe: [PATCH v3 8/8] nvdimm: Fix firmware activation deadlock scenarios
On Fri, Apr 22, 2022 at 9:28 PM Ira Weiny <ira.weiny@intel.com> wrote:
>
> On Thu, Apr 21, 2022 at 08:33:51AM -0700, Dan Williams wrote:
> > Lockdep reports the following deadlock scenarios for CXL root device
> > power-management, device_prepare(), operations, and device_shutdown()
> > operations for 'nd_region' devices:
> >
> > ---
> > Chain exists of:
> > &nvdimm_region_key --> &nvdimm_bus->reconfig_mutex --> system_transition_mutex
> >
> > Possible unsafe locking scenario:
> >
> > CPU0 CPU1
> > ---- ----
> > lock(system_transition_mutex);
> > lock(&nvdimm_bus->reconfig_mutex);
> > lock(system_transition_mutex);
> > lock(&nvdimm_region_key);
> >
> > --
> >
> > Chain exists of:
> > &cxl_nvdimm_bridge_key --> acpi_scan_lock --> &cxl_root_key
> >
> > Possible unsafe locking scenario:
> >
> > CPU0 CPU1
> > ---- ----
> > lock(&cxl_root_key);
> > lock(acpi_scan_lock);
> > lock(&cxl_root_key);
> > lock(&cxl_nvdimm_bridge_key);
> >
> > ---
> >
> > These stem from holding nvdimm_bus_lock() over hibernate_quiet_exec()
> > which walks the entire system device topology taking device_lock() along
> > the way. The nvdimm_bus_lock() is protecting against unregistration,
> > multiple simultaneous ops callers, and preventing activate_show() from
> > racing activate_store(). For the first 2, the lock is redundant.
> > Unregistration already flushes all ops users, and sysfs already prevents
> > multiple threads to be active in an ops handler at the same time. For
> > the last userspace should already be waiting for its last
> > activate_store() to complete, and does not need activate_show() to flush
> > the write side, so this lock usage can be deleted in these attributes.
> >
>
> I'm sorry if this is obvious but why can't the locking be removed from
> capability_show() and nvdimm_bus_firmware_visible() as well?

It can, that's a good catch, thanks.

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