Re: [Xen-devel] [PATCH v2] rwlock: allow recursive read locking when already locked in write mode

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 21, 2020 at 03:41:59PM +0100, Jan Beulich wrote:
> On 21.02.2020 15:26, Roger Pau Monné wrote:
> > On Fri, Feb 21, 2020 at 02:36:52PM +0100, Jan Beulich wrote:
> >> On 21.02.2020 10:10, Roger Pau Monné wrote:
> >>> On Thu, Feb 20, 2020 at 07:20:06PM +0000, Julien Grall wrote:
> >>>> Hi,
> >>>>
> >>>> On 20/02/2020 17:31, Roger Pau Monne wrote:
> >>>>> Allow a CPU already holding the lock in write mode to also lock it in
> >>>>> read mode. There's no harm in allowing read locking a rwlock that's
> >>>>> already owned by the caller (ie: CPU) in write mode. Allowing such
> >>>>> accesses is required at least for the CPU maps use-case.
> >>>>>
> >>>>> In order to do this reserve 14bits of the lock, this allows to support
> >>>>> up to 16384 CPUs. Also reduce the write lock mask to 2 bits: one to
> >>>>> signal there are pending writers waiting on the lock and the other to
> >>>>> signal the lock is owned in write mode. Note the write related data
> >>>>> is using 16bits, this is done in order to be able to clear it (and
> >>>>> thus release the lock) using a 16bit atomic write.
> >>>>>
> >>>>> This reduces the maximum number of concurrent readers from 16777216 to
> >>>>> 65536, I think this should still be enough, or else the lock field
> >>>>> can be expanded from 32 to 64bits if all architectures support atomic
> >>>>> operations on 64bit integers.
> >>>>
> >>>> FWIW, arm32 is able to support atomic operations on 64-bit integers.
> >>>>
> >>>>>   static inline void _write_unlock(rwlock_t *lock)
> >>>>>   {
> >>>>> -    /*
> >>>>> -     * If the writer field is atomic, it can be cleared directly.
> >>>>> -     * Otherwise, an atomic subtraction will be used to clear it.
> >>>>> -     */
> >>>>> -    atomic_sub(_QW_LOCKED, &lock->cnts);
> >>>>> +    /* Since the writer field is atomic, it can be cleared directly. */
> >>>>> +    ASSERT(_is_write_locked_by_me(atomic_read(&lock->cnts)));
> >>>>> +    BUILD_BUG_ON(_QR_SHIFT != 16);
> >>>>> +    write_atomic((uint16_t *)&lock->cnts, 0);
> >>>>
> >>>> I think this is an abuse to cast an atomic_t() directly into a uint16_t. 
> >>>> You
> >>>> would at least want to use &lock->cnts.counter here.
> >>>
> >>> Sure, I was wondering about this myself.
> >>>
> >>> Will wait for more comments, not sure whether this can be fixed upon
> >>> commit if there are no other issues.
> >>
> >> It's more than just adding another field specifier here. A cast like
> >> this one is endianness-unsafe, and hence a trap waiting for a big
> >> endian port attempt to fall into. At the very least this should cause
> >> a build failure on big endian systems, even better would be if it was
> >> endianness-safe.
> > 
> > Why don't we leave the atomic_dec then?
> 
> Because you need to invoke smp_processor_id() to calculate the value
> to use in the subtraction. I'm not meaning to say I'm entirely
> opposed (seeing how much of a discussion we're having), but the
> "simple write of zero" approach is certainly appealing.

Well, we could avoid the smp_processor_id() call and instead use:

atomic_sub(atomic_read(&lock->cnts) & 0xffff, &lock->cnts);

Note mask is using the low 16bits now, but if we go the atomic_sub
route we could change the CPU ID fields to be 12 bits again and thus
have some more room for the readers count.

Thanks, Roger.

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