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 {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct ck_epoch e_epoch __ali=
gned(EPOCH_ALIGN);
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 epoch_record_t e_pcpu_record;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 int=C2=A0=C2=A0=C2=A0=C2=A0 e=
_in_use;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 int=C2=A0=C2=A0=C2=A0=C2=A0 e=
_flags;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct sx e_drain_sx;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct mtx e_drain_mtx;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 volatile int e_drain_count;
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 const char *e_name;
};
The e_drain_sx and e_drain_mtx are only used in
void
epoch_drain_callbacks(epoch_t epoch)
{
...
=C2=A0=C2=A0=C2=A0 DROP_GIANT();
=C2=A0=C2=A0=C2=A0 sx_xlock(&epoch->e_drain_sx);
=C2=A0=C2=A0=C2=A0 mtx_lock(&epoch->e_drain_mtx);
...
=C2=A0=C2=A0=C2=A0 mtx_unlock(&epoch->e_drain_mtx);
=C2=A0=C2=A0=C2=A0 sx_xunlock(&epoch->e_drain_sx);
=C2=A0=C2=A0=C2=A0 PICKUP_GIANT();
}
Why is there a combination of a shared/exclusive lock and a mutex used=20
like this? Why is a single mutex insufficient?
--=20
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