Re: [PATCH v2 4/4] powerpc/rtas: upgrade internal arch spinlocks

[Andrew Donnellan <ajd@linux.ibm.com>]

On Tue, 2023-01-24 at 08:04 -0600, Nathan Lynch wrote:
> At the time commit f97bb36f705d ("powerpc/rtas: Turn rtas lock into a
> raw spinlock") was written, the spinlock lockup detection code called
> __delay(), which will not make progress if the timebase is not
> advancing. Since the interprocessor timebase synchronization sequence
> for chrp, cell, and some now-unsupported Power models can temporarily
> freeze the timebase through an RTAS function (freeze-time-base), the
> lock that serializes most RTAS calls was converted to arch_spinlock_t
> to prevent kernel hangs in the lockup detection code.
> 
> However, commit bc88c10d7e69 ("locking/spinlock/debug: Remove
> spinlock
> lockup detection code") removed that inconvenient property from the
> lock debug code several years ago. So now it should be safe to
> reintroduce generic locks into the RTAS support code, primarily to
> increase lockdep coverage.
> 
> Making rtas_lock a spinlock_t would violate lock type nesting rules
> because it can be acquired while holding raw locks, e.g. pci_lock and
> irq_desc->lock. So convert it to raw_spinlock_t. There's no apparent
> reason not to upgrade timebase_lock as well.
> 
> Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>

I'm no locking expert but this looks reasonable from a quick read-
through.

Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>

-- 
Andrew Donnellan    OzLabs, ADL Canberra
ajd@linux.ibm.com   IBM Australia Limited