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 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?

The ASSERT I've added can be turned into a BUG_ON, and that's likely
even safer than what we currently have.

Thanks, Roger.

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