Does Monitor.Wait ensure that fields are re-read?
Posted
by Marc Gravell
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Published on 2010-03-12T07:57:31Z
Indexed on
2010/03/12
8:07 UTC
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It is generally accepted (I believe!) that a lock
will force any values from fields to be reloaded (essentially acting as a memory-barrier or fence - my terminology in this area gets a bit loose, I'm afraid), with the consequence that fields that are only ever accessed inside a lock
do not themselves need to be volatile
.
(If I'm wrong already, just say!)
A good comment was raised here, questioning whether the same is true if code does a Wait()
- i.e. once it has been Pulse()
d, will it reload fields from memory, or could they be in a register (etc).
Or more simply: does the field need to be volatile
to ensure that the current value is obtained when resuming after a Wait()
?
Looking at reflector, Wait
calls down into ObjWait
, which is managed internalcall
(the same as Enter
).
The scenario in question was:
bool closing;
public bool TryDequeue(out T value) {
lock (queue) { // arbitrary lock-object (a private readonly ref-type)
while (queue.Count == 0) {
if (closing) { // <==== (2) access field here
value = default(T);
return false;
}
Monitor.Wait(queue); // <==== (1) waits here
}
...blah do something with the head of the queue
}
}
Obviously I could just make it volatile
, or I could move this out so that I exit and re-enter the Monitor
every time it gets pulsed, but I'm intrigued to know if either is necessary.
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