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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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