Re: [Xen-devel] [PATCH 1/2] smp: convert the cpu maps lock into a rw lock

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Feb 19, 2020 at 02:42:58PM +0100, Jan Beulich wrote:
> On 19.02.2020 14:19, Roger Pau Monné wrote:
> > On Wed, Feb 19, 2020 at 01:56:02PM +0100, Jan Beulich wrote:
> >> On 13.02.2020 12:32, Roger Pau Monne wrote:
> >>>  void __init register_cpu_notifier(struct notifier_block *nb)
> >>>  {
> >>> -    if ( !spin_trylock(&cpu_add_remove_lock) )
> >>> +    if ( !write_trylock(&cpu_add_remove_lock) )
> >>>          BUG(); /* Should never fail as we are called only during boot. */
> >>>      notifier_chain_register(&cpu_chain, nb);
> >>> -    spin_unlock(&cpu_add_remove_lock);
> >>> +    write_unlock(&cpu_add_remove_lock);
> >>>  }
> >>
> >> So why a write lock here?
> > 
> > notifier_chain_register calls cannot be made in parallel, as they
> > modify the underlying notifier list without taking any additional
> > locks.
> 
> I.e. the lock is being (ab)used to also protect the notifier list,
> which is certainly not its purpose. (The locking seems quite
> pointless here anyway considering the __init together with the
> nature of the function.)

Right, it's quite likely that the lock is pointless, I haven't looked
at all the callers to assure this. Anyway, iff the lock can be safely
removed that should be done in a different patch, and not this one
IMO. This merely switching existing users to use a rw lock.

Thanks, Roger.

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