2edc4d4634
Currently, startpanic_m (which prepares for an unrecoverable panic) goes out of its way to make it possible to allocate during panic handling by allocating an mcache if there isn't one. However, this is both potentially dangerous and unnecessary. Allocating an mcache is a generally complex thing to do in an already precarious situation. Specifically, it requires obtaining the heap lock, and there's evidence that this may be able to deadlock (#23360). However, it's also unnecessary because we never allocate from the unrecoverable panic path. This didn't use to be the case. The call to allocmcache was introduced long ago, in CL 7388043, where it was in preparation for separating Ms and Ps and potentially running an M without an mcache. At the time, after calling startpanic, the runtime could call String and Error methods on panicked values, which could do anything including allocating. That was generally unsafe even at the time, and CL 19792 fixed this be pre-printing panic messages before calling startpanic. As a result, we now no longer allocate after calling startpanic. This CL not only removes the allocmcache call, but goes a step further to explicitly disallow any allocation during unrecoverable panic handling, even in situations where it might be safe. This way, if panic handling ever does an allocation that would be unsafe in unusual circumstances, we'll know even if it happens during normal circumstances. This would help with debugging #23360, since the deadlock in allocmcache is currently masking the real failure. Beyond all.bash, I manually tested this change by adding panics at various points in early runtime init, signal handling, and the scheduler to check unusual panic situations. Change-Id: I85df21e2b4b20c6faf1f13fae266c9339eebc061 Reviewed-on: https://go-review.googlesource.com/88835 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> |
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