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

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

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.

Thanks, Roger.

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