Issue 142: Stopping preemption causes null reference exception
http://code.google.com/p/v8/issues/detail?id=142
New issue report by jcpstuff:
Using several threads and calling Start/Stop preemption works well for the
most part. Sometimes what will happen, though, is a null deferencing in
ContextSwitcher::PreemptionReceived(). After tracing the execution, this
seems to be what happens:
Thread A calls StartPreemption and begins executing Javascript.
The ContextSwitcher thread C spins up.
Thread B calls StopPreemption (for another reason I will add another bug
for; namely, Context creation is not preemption-safe).
Thread B, in ContextSwitcher::Stop, sets keep_going_ to false and signals
the preemption semaphore.
Thread C is done with its wait time, signals preemption, waits for the
semaphore (which is already signaled), sees that keep_going_ is false, and
exits. The preemption flag has been set, though.
Thread A resumes execution and finds that it has been preempted. It calls
ContextSwitcher::PreemptionReceived.
Thread A dereferences the switcher variable, even though it has been
nulled out in ContextSwitcher::StopPreemption.
To fix, switcher needs to be marked as volatile.
ContextSwitcher::PreemptionReceived needs to check that switcher != null,
and the implementation of StopPreemptionneeds to set switcher to null
before deleting it.
Issue attributes:
Status: New
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