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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.02.2020 14:46, 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?

Well, a union (with the second half being an array of uint16_t) could
be the mechanism to implement this in an endian safe way, but the
union alone won't help - you'd still have to decide which of the array
elements you actually need to write to. And of course you'd then also
make assumptions on sizeof(int) which are more strict than our current
ones (which I'd like to avoid if possible).

Jan

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