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The Go programming language
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The implementation and use patterns of onM assume that they run on either the m->curg or m->g0 stack. Calling onM from m->gsignal has two problems: (1) When not on g0, onM switches to g0 and then "back" to curg. If we didn't start at curg, bad things happen. (2) The use of scalararg/ptrarg to pass C arguments and results assumes that there is only one onM call at a time. If a gsignal starts running, it may have interrupted the setup/teardown of the args for an onM on the curg or g0 stack. Using scalararg/ptrarg itself would smash those. We can fix (1) by remembering what g was running before the switch. We can fix (2) by requiring that uses of onM that might happen on a signal handling stack must save the old scalararg/ptrarg and restore them after the call, instead of zeroing them. The only sane way to do this is to introduce a separate onM_signalsafe that omits the signal check, and then if you see a call to onM_signalsafe you know the surrounding code must preserve the old scalararg/ptrarg values. (The implementation would be that onM_signalsafe just calls fn if on the signal stack or else jumps to onM. It's not necessary to have two whole copies of the function.) (2) is not a problem if the caller and callee are both Go and a closure is used instead of the scalararg/ptrarg slots. For now, I think we can avoid calling onM from code executing on gsignal stacks, so just reject it. In the long term, (2) goes away (as do the scalararg/ptrarg slots) once everything is in Go, and at that point fixing (1) would be trivial and maybe worth doing just for regularity. LGTM=iant R=golang-codereviews, iant CC=dvyukov, golang-codereviews, khr, r https://golang.org/cl/135400043 |
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