60ef4b2c1e
CL 424257 modified gentraceback to switch gp when jumping from a system stack to a user stack to simplify reasoning through the rest of the function. This has the unintended side-effect of also switching all references to gp.m. The vast majority of the time, g0.m and curg.m are the same across a stack switch, making this a no-op, but there's at least one case where this isn't true: if a profiling signal happens in execute between setting mp.curg and setting gp.m. In this case, mp.curg.m is briefly nil, which can cause gentraceback to crash with a nil pointer dereference. We see this failure (surprisingly frequently!) in profiling tests in the morestack=mayMoreStackPreempt testing mode (#48297). Fix this by making only jumping stacks if doing so will not switch Ms. This restores the original property that gp.m doesn't change across the stack jump, and makes gentraceback a little more conservative about jumping stacks. Fixes #54885. Change-Id: Ib1524c41c748eeff35896e0f3abf9a7efbe5969f Reviewed-on: https://go-review.googlesource.com/c/go/+/428656 Reviewed-by: Michael Pratt <mpratt@google.com> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Auto-Submit: Austin Clements <austin@google.com> |
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