Why is there e_drain_sx and e_drain_mtx?

[Sebastian Huber]

On 29/12/2020 12:07, Hans Petter Selasky wrote:

> On 12/29/20 11:49 AM, Sebastian Huber wrote:
>> Hello,
>>
>> in the epoch based reclamation implementation we have
>>
>> struct epoch {
>>          struct ck_epoch e_epoch __aligned(EPOCH_ALIGN);
>>          epoch_record_t e_pcpu_record;
>>          int     e_in_use;
>>          int     e_flags;
>>          struct sx e_drain_sx;
>>          struct mtx e_drain_mtx;
>>          volatile int e_drain_count;
>>          const char *e_name;
>> };
>>
>> The e_drain_sx and e_drain_mtx are only used in
>>
>> void
>> epoch_drain_callbacks(epoch_t epoch)
>> {
>> ...
>>      DROP_GIANT();
>>
>>      sx_xlock(&epoch->e_drain_sx);
>>      mtx_lock(&epoch->e_drain_mtx);
>>
>> ...
>>
>>      mtx_unlock(&epoch->e_drain_mtx);
>>      sx_xunlock(&epoch->e_drain_sx);
>>
>>      PICKUP_GIANT();
>> }
>>
>> Why is there a combination of a shared/exclusive lock and a mutex 
>> used like this? Why is a single mutex insufficient?
>>
>
> Hi Sebastian,
>
> The sx_xlock() is there because the operation may sleep and mutexes 
> must be dropped when sleeping. We only allow one drain at a time.
>
> The mtx_lock() is there to make the operation atomic with regards to 
> the msleep/wakeup sequence. Search for the use of e_drain_mtx . Else 
> the msleep and wakeup calls may miss eachother.
Thanks for the explanation. I overlooked the msleep() call.
>
> How is the porting going otherwise? Do you see any performance 
> improvements using EPOCH?
It works quite well, even on old chips like the MCF5475. I didn't 
monitor the performance over time, but nobody complained so far.

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