Re: [Xen-devel] [PATCH] rwlock: allow recursive read locking when already locked in write mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.02.2020 15:11, Roger Pau Monné wrote:
> On Thu, Feb 20, 2020 at 01:48:54PM +0100, Jan Beulich wrote:
>> Another option is to use the recurse_cpu field of the
>> associated spin lock: The field is used for recursive locks
>> only, and hence the only conflict would be with
>> _spin_is_locked(), which we don't (and in the future then
>> also shouldn't) use on this lock.
> 
> I looked into that also, but things get more complicated AFAICT, as it's
> not possible to atomically fetch the state of the lock and the owner
> CPU at the same time. Neither you could set the LOCKED bit and the CPU
> at the same time.

There's no need to atomically fetch both afaics: The field is
valid only if the LOCKED bit is set. And when reading the
field you only care about the value being equal to
smp_processor_id(), i.e. it is fine to set LOCKED before
updating the CPU field on lock, and to reset the CPU field to
SPINLOCK_NO_CPU (or whatever it's called) before clearing
LOCKED.

Jan

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