summaryrefslogtreecommitdiff
path: root/lib/thread
diff options
context:
space:
mode:
authorOri Bernstein <ori@eigenstate.org>2015-09-16 14:42:00 -0400
committerOri Bernstein <ori@eigenstate.org>2015-09-16 14:42:00 -0400
commitd3091cbc968337bdd47efe3bab7e752972cad726 (patch)
tree2d1b7624761a02753732d2753fc7be421b4dea69 /lib/thread
parent1edee0e32036f4d1cb23e94cab7b8f72f6ee9193 (diff)
downloadmc-d3091cbc968337bdd47efe3bab7e752972cad726.tar.gz
Comment the code.
Diffstat (limited to 'lib/thread')
-rw-r--r--lib/thread/mutex+linux.myr44
1 files changed, 32 insertions, 12 deletions
diff --git a/lib/thread/mutex+linux.myr b/lib/thread/mutex+linux.myr
index 62aa88a..71c7301 100644
--- a/lib/thread/mutex+linux.myr
+++ b/lib/thread/mutex+linux.myr
@@ -16,7 +16,7 @@ pkg thread =
const Unlocked = 0
const Locked = 1
-const Lockedcontended = 2
+const Contended = 2
generic Zptr = 0 castto(@a#)
var nspin = 1 /* FIXME: pick a sane number, based on CPU count */
@@ -27,7 +27,9 @@ const mkmtx = {
const mtxlock = {mtx
var c
- /* uncontended case: we get an unlocked mutex, and we lock it */
+ /*
+ Uncontended case: we get an unlocked mutex, and we lock it.
+ */
for var i = 0; i < nspin; i++
c = xcas(&mtx._state, Unlocked, Locked)
if c == Unlocked
@@ -36,14 +38,18 @@ const mtxlock = {mtx
relax()
;;
- /* contended: we set the lock _state to sleep */
+ /*
+ Contended case: we set the lock state to Contended. This indicates that there
+ the lock is locked, and we potentially have threads waiting on it, which means
+ that we will need to wake them up.
+ */
if c == Locked
- c = xchg(&mtx._state, Lockedcontended)
+ c = xchg(&mtx._state, Contended)
;;
while c != Unlocked
- sys.futex(&mtx._state, sys.Futexwait | sys.Futexpriv, Lockedcontended, Zptr, Zptr, 0)
- c = xchg(&mtx._state, Lockedcontended)
+ sys.futex(&mtx._state, sys.Futexwait | sys.Futexpriv, Contended, Zptr, Zptr, 0)
+ c = xchg(&mtx._state, Contended)
;;
}
@@ -52,17 +58,30 @@ const mtxtrylock = {mtx
}
const mtxunlock = {mtx
- /* uncontended sleep means we can just unlock and move on */
- if mtx._state == Lockedcontended
- mtx._state = Unlocked
- elif xchg(&mtx._state, Unlocked) == Locked
+ /*
+ Uncontended case: If the mutex state is not contended, and we still
+ are uncontended by the xchg() call, then it's safe to simply return;
+ nobody was waiting for us.
+ */
+ if mtx._state != Contended && xchg(&mtx._state, Unlocked) == Locked
->
;;
+ /*
+ Contended case: Now we unlock, and wait a little bit to see if anyone
+ takes the lock. If someone takes the lock, we can avoid waking entering
+ the kernel to wake processes. However, we need to set the lock to contended
+ so that if there are still waiters, the thread that took the lock takes
+ responsibility for unlocking.
+ */
+ mtx._state = Unlocked
for var i = 0; i < nspin; i++
if mtx._state != Unlocked
- /* there might have been waiters, but we set the _state to unlocked */
- if xcas(&mtx._state, Locked, Lockedcontended) == Contendedlock
+ /*
+ If we're locked, set the state to contended to avoid
+ missed wakes.
+ */
+ if xcas(&mtx._state, Locked, Contended) == Contended
->
;;
;;
@@ -73,4 +92,5 @@ const mtxunlock = {mtx
}
const relax = {
+ /* FIXME: what is an appropriate way to kill CPU time without entering kernel? */
}