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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.02.2020 15:06, Jürgen Groß wrote:
> On 21.02.20 14:49, Julien Grall wrote:
>>
>>
>> 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.
> 
> union rwlockword {
>      atomic_t cnts;
>      uint32_t val;
>      struct {
>          uint16_t write;
>          uint16_t readers;
>      };
> };
> 
> static inline const uint32_t _qr_bias(
>      const union rwlockword {
>          .write = 0,
>          .readers = 1
>      } x;
> 
>      return x.val;
> }
> 
> ...
>      cnts = atomic_add_return(_qr_bias(), &lock->cnts);
> ...
> 
> I guess this should do the trick, no?

I'm afraid it won't, and not just because of the sizeof() aspect
already pointed out. Your x variable would end up like this in
memory:

little: 00 00 01 00
big:    00 00 00 01 => 00000001

which, read as 32-bit value, then ends up being

little: 00010000
big:    00000001

The add therefore would be able to spill into the high 16 bits.

Jan

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