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asyncevents: fix missing GC root and race #44956

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merged 1 commit into from
Apr 13, 2022
Merged

asyncevents: fix missing GC root and race #44956

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67 changes: 42 additions & 25 deletions base/asyncevent.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -45,13 +45,22 @@ the async condition object itself.
"""
function AsyncCondition(cb::Function)
async = AsyncCondition()
t = @task while _trywait(async)
cb(async)
isopen(async) || return
t = @task begin
unpreserve_handle(async)
Comment on lines 47 to +49
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Isn't async reachable through t.code? Why would it be finalized in the old code?

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t is only reachable through async

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ahhhh

while _trywait(async)
cb(async)
isopen(async) || return
end
end
# here we are mimicking parts of _trywait, in coordination with task `t`
preserve_handle(async)
@lock async.cond begin
if async.set
schedule(t)
else
_wait2(async.cond, t)
end
end
lock(async.cond)
_wait2(async.cond, t)
unlock(async.cond)
return async
end

Expand Down Expand Up @@ -115,6 +124,7 @@ function _trywait(t::Union{Timer, AsyncCondition})
# full barrier now for AsyncCondition
t isa Timer || Core.Intrinsics.atomic_fence(:acquire_release)
else
t.isopen || return false
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@tkf tkf Apr 13, 2022 •
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Since this is outside a lock, don't we need to acquire load/release store t.isopen for AsyncCondition so that close(async) happens-before wait(async) throws? Not sure if that's needed though.

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@tkf tkf Apr 13, 2022 •
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I think it's conceivable that someone writes

async = AsyncCondition()

t1 = @spawn begin  # code in external library
    f1()
    ccall(:uv_async_send, Cvoid, (Ptr{Cvoid},), async)
end

t2 = @spawn begin
    try
        wait(async)
    catch err
        err isa EOFError || rethrow()
    end
    close(async)
    f2()
end

t3 = @spawn begin
    try
        wait(async)
    catch err
        err isa EOFError || rethrow()
    end
    close(async)
    f3()
end

and expect f1 to happen-before f2 and f3. There are better ways to do level-triggering based on AsyncCondition but it does not look super crazy to me.

But I'm also OK with documenting that wait does not establish any ordering if it throws.

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Establishing happens-before seems like a good idea. We don't do that currently, so it is not relevant to this PR however.

t.handle == C_NULL && return false
iolock_begin()
set = t.set
Expand All @@ -123,14 +133,12 @@ function _trywait(t::Union{Timer, AsyncCondition})
lock(t.cond)
try
set = t.set
if !set
if t.handle != C_NULL
iolock_end()
set = wait(t.cond)
unlock(t.cond)
iolock_begin()
lock(t.cond)
end
if !set && t.isopen && t.handle != C_NULL
iolock_end()
set = wait(t.cond)
unlock(t.cond)
iolock_begin()
lock(t.cond)
end
finally
unlock(t.cond)
Expand Down Expand Up @@ -266,19 +274,28 @@ julia> begin
"""
function Timer(cb::Function, timeout::Real; interval::Real=0.0)
timer = Timer(timeout, interval=interval)
t = @task while _trywait(timer)
try
cb(timer)
catch err
write(stderr, "Error in Timer:\n")
showerror(stderr, err, catch_backtrace())
return
t = @task begin
unpreserve_handle(timer)
while _trywait(timer)
try
cb(timer)
catch err
write(stderr, "Error in Timer:\n")
showerror(stderr, err, catch_backtrace())
return
end
isopen(timer) || return
end
end
# here we are mimicking parts of _trywait, in coordination with task `t`
preserve_handle(timer)
@lock timer.cond begin
if timer.set
schedule(t)
else
_wait2(timer.cond, t)
end
isopen(timer) || return
end
lock(timer.cond)
_wait2(timer.cond, t)
unlock(timer.cond)
return timer
end

Expand Down