> On 21 Jun 2021, at 17:32, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Mon, Jun 21, 2021 at 03:30:14PM +0000, Haakon Bugge wrote:
>>
>>
>>> On 21 Jun 2021, at 16:35, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>>>
>>> On Wed, Jun 16, 2021 at 04:35:53PM +0200, Håkon Bugge wrote:
>>>> +#define BIT_ACCESS_FUNCTIONS(b) \
>>>> + static inline void set_##b(unsigned long flags) \
>>>> + { \
>>>> + /* set_bit() does not imply a memory barrier */ \
>>>> + smp_mb__before_atomic(); \
>>>> + set_bit(b, &flags); \
>>>> + /* set_bit() does not imply a memory barrier */ \
>>>> + smp_mb__after_atomic(); \
>>>> + }
>>>
>>> This isn't needed, set_bit/test_bit are already atomic with
>>> themselves, we should not need to introduce release semantics.
>>
>> They are atomic, yes. But set_bit() does not provide a memory barrier (on x86_64, yes, but not as per the Linux definition of set_bit()).
>>
>> We have (paraphrased):
>>
>> id_priv->min_rnr_timer = min_rnr_timer;
>> set_bit(MIN_RNR_TIMER_SET, &id_priv->flags);
>>
>> Since set_bit() does not provide a memory barrier, another thread
>> may observe the MIN_RNR_TIMER_SET bit in id_priv->flags, but the
>> id_priv->min_rnr_timer value is not yet globally visible. Hence,
>> IMHO, we need the memory barriers.
>
> No, you need proper locks.
Either will work in my opinion. If you prefer locking, I can do that. This is not performance critical.
Thxs, Håkon
>
> Jason
