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runtime: doubly fix "double wakeup" panic
runtime.gchelper depends on the non-atomic load of work.ndone happening strictly before the atomic add of work.nwait. Until very recently (commit978af9c2db
, fixing #20334), the compiler reordered these operations. This created a race since work.ndone can change as soon as work.nwait is equal to work.ndone. If that happened, more than one gchelper could attempt to wake up the work.alldone note, causing a "double wakeup" panic. This was fixed in the compiler, but to make this code less subtle, make the load of work.ndone atomic. This clearly forces the order of these operations, ensuring the race doesn't happen. Fixes #19305 (though really978af9c2db
fixed it). Change-Id: Ieb1a84e1e5044c33ac612c8a5ab6297e7db4c57d Reviewed-on: https://go-review.googlesource.com/43311 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>
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@ -2105,7 +2105,7 @@ func gchelper() {
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traceGCScanDone()
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}
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nproc := work.nproc // work.nproc can change right after we increment work.ndone
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nproc := atomic.Load(&work.nproc) // work.nproc can change right after we increment work.ndone
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if atomic.Xadd(&work.ndone, +1) == nproc-1 {
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notewakeup(&work.alldone)
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}
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