Why is there e_drain_sx and e_drain_mtx?

[Sebastian Huber]

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?

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