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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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