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task: schedule sticky tasks correctly with _wait2 #42750

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Oct 22, 2021
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task: schedule sticky tasks correctly with _wait2 #42750

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task: schedule sticky tasks correctly with _wait2
vtjnash Oct 21, 2021
088cd6f
Update base/task.jl
vtjnash Oct 21, 2021
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11 changes: 11 additions & 0 deletions base/condition.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,17 @@ function _wait2(c::GenericCondition, waiter::Task)
ct = current_task()
assert_havelock(c)
push!(c.waitq, waiter)
# since _wait2 is similar to schedule, we should observe the sticky bit now
if waiter.sticky && Threads.threadid(waiter) == 0
# Issue #41324
# t.sticky && tid == 0 is a task that needs to be co-scheduled with
# the parent task. If the parent (current_task) is not sticky we must
# set it to be sticky.
# XXX: Ideally we would be able to unset this
ct.sticky = true
tid = Threads.threadid()
ccall(:jl_set_task_tid, Cvoid, (Any, Cint), waiter, tid-1)
end
return
end

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13 changes: 13 additions & 0 deletions base/task.jl
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Expand Up @@ -306,6 +306,18 @@ function _wait2(t::Task, waiter::Task)
if !istaskdone(t)
push!(t.donenotify.waitq, waiter)
unlock(t.donenotify)
# since _wait2 is similar to schedule, we should observe the sticky
# bit, even if we aren't calling `schedule` due to this early-return
if waiter.sticky && Threads.threadid(waiter) == 0
# Issue #41324
# t.sticky && tid == 0 is a task that needs to be co-scheduled with
# the parent task. If the parent (current_task) is not sticky we must
# set it to be sticky.
# XXX: Ideally we would be able to unset this
current_task().sticky = true
tid = Threads.threadid()
ccall(:jl_set_task_tid, Cvoid, (Any, Cint), waiter, tid-1)
Comment on lines +317 to +319
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@kpamnany kpamnany May 1, 2023

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@vtjnash and @vchuravy: I think this is wrong. Here, waiter is waiting on t, not on current_task() which can be completely different (e.g. see bind).

Apparently, we want to pin t and waiter to the same thread. But I'm not sure how this follows from the original problem: #41324. It's perfectly reasonable for a task in the interactive threadpool to wait on a task in the default threadpool, but this makes that unachievable.

I know it's been a while, but can either of you remember why this was added after #41334?

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@vtjnash vtjnash May 1, 2023 •
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_wait2 is defined as semantically a call to schedule the Task waiter to immediately call wait on some condition variable t. Hence why it has the same side-effects as calling schedule: it should pin current_task and waiter to one thread, but not t (which will have completed before waiter runs anyways).

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@kpamnany kpamnany May 1, 2023

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That task t will have completed before waiter runs makes sense. But this _wait2 as opposed to the one in condition.jl has nothing to do with Conditions. t is a Task here, not a Condition. The semantics of this _wait2 seem unrelated to current_task().

end
return nothing
else
unlock(t.donenotify)
Expand Down Expand Up @@ -455,6 +467,7 @@ function errormonitor(t::Task)
end
nothing
end
t2.sticky = false
_wait2(t, t2)
return t
end
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