runtime: runqputbatch does not protect its call to globrunqputbatch

[mknyszek]

As part of https://go-review.googlesource.com/c/go/+/216198 which landed in the Go 1.15 cycle, the new function runqputbatch documents that it may acquire sched.lock because it needs to call globrunqputbatch, but it never does.

Not protecting globrunqputbatch can lead to very bad things, since manipulating the global runq is protected by sched.lock. In particular, we've seen nil g pointers in the global runq as well as general corruption of the global runq field in production.

The fix is straightforward and I'll upload it shortly. This problem is new in Go 1.15.

[mknyszek]

CC @dmitshur @prattmic @ianlancetaylor

[gopherbot]

Change https://golang.org/cl/245377 mentions this issue: runtime: hold sched.lock over globrunqputbatch in runqputbatch

[ianlancetaylor]

Bother, sorry about that. Not sure how that happened, and not sure how no test caught it.