Date: Tue, 29 Dec 2020 11:49:49 +0100 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: freebsd-hackers@freebsd.org Subject: Why is there e_drain_sx and e_drain_mtx? Message-ID: <5eeae691-b7d4-932b-14cc-065a368e77de@embedded-brains.de>
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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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