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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.)

Jan

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