ACPI: NFIT: fix a potential deadlock during NFIT teardown

Lockdep reports that acpi_nfit_shutdown() may deadlock against an
opportune acpi_nfit_scrub(). acpi_nfit_scrub () is run from inside a
'work' and therefore has already acquired workqueue-internal locks. It
also acquiires acpi_desc->init_mutex. acpi_nfit_shutdown() first
acquires init_mutex, and was subsequently attempting to cancel any
pending workqueue items. This reversed locking order causes a potential
deadlock:

    ======================================================
    WARNING: possible circular locking dependency detected
    6.2.0-rc3 torvalds#116 Tainted: G           O     N
    ------------------------------------------------------
    libndctl/1958 is trying to acquire lock:
    ffff888129b461c0 ((work_completion)(&(&acpi_desc->dwork)->work)){+.+.}-{0:0}, at: __flush_work+0x43/0x450

    but task is already holding lock:
    ffff888129b460e8 (&acpi_desc->init_mutex){+.+.}-{3:3}, at: acpi_nfit_shutdown+0x87/0xd0 [nfit]

    which lock already depends on the new lock.

    ...

    Possible unsafe locking scenario:

          CPU0                    CPU1
          ----                    ----
     lock(&acpi_desc->init_mutex);
                                  lock((work_completion)(&(&acpi_desc->dwork)->work));
                                  lock(&acpi_desc->init_mutex);
     lock((work_completion)(&(&acpi_desc->dwork)->work));

    *** DEADLOCK ***

Since the workqueue manipulation is protected by its own internal locking,
the cancellation of pending work doesn't need to be done under
acpi_desc->init_mutex. Move cancel_delayed_work_sync() outside the
init_mutex to fix the deadlock. Any work that starts after
acpi_nfit_shutdown() drops the lock will see ARS_CANCEL, and the
cancel_delayed_work_sync() will safely flush it out.

Reported-by: Dan Williams <[email protected]>
Signed-off-by: Vishal Verma <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dan Williams <[email protected]>

Author: stellarhopper

drivers/acpi/nfit/core.c

@@ -3297,8 +3297,8 @@ void acpi_nfit_shutdown(void *data)
 
 	mutex_lock(&acpi_desc->init_mutex);
 	set_bit(ARS_CANCEL, &acpi_desc->scrub_flags);
-	cancel_delayed_work_sync(&acpi_desc->dwork);
 	mutex_unlock(&acpi_desc->init_mutex);
+	cancel_delayed_work_sync(&acpi_desc->dwork);
 
 	/*
 	 * Bounce the nvdimm bus lock to make sure any in-flight