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

[Julien Grall <julien@xxxxxxx>]

On 21/02/2020 13:46, Jürgen Groß wrote:
> On 21.02.20 14:36, 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.
> Wouldn't a union be the better choice?
You would not be able to use atomic_t in that case as you can't assume 
the layout of the structure.
Cheers,

--
Julien Grall

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