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acquireLockRank and releaseLockRank are called from nosplit context, and thus must be nosplit. lockWithRank, unlockWithRank, and lockWithRankMayAcquire are called from spittable context, and thus don't strictly need to be nosplit. The stated reasoning for making these functions nosplit is to avoid re-entrant calls due to a stack split on function entry taking a lock. There are two potential issues at play here: 1. A stack split on function entry adds a new lock ordering edge before we (a) take lock l, or (b) release lock l. 2. A stack split in a child call (such as to lock2) introduces a new lock ordering edge _in the wrong order_ because e.g., in the case of lockWithRank, we've noted that l is taken, but the stack split in lock2 actually takes stack split locks _before_ l is actually locked. (1) is indeed avoided by marking these functions nosplit, but this is really just a bit of duct tape that generally has no effect overall. Any earlier call can have a stack split and introduce the same new edge. This includes lock/unlock which are not nosplit! I began this CL as a change to extend nosplit to lock and unlock to try to make this mitigation more effective, but I've realized that as long as there is a _single_ nosplit call between a lock and unlock, we can end up with the edge. There seems to be few enough cases without any calls that is does not seem worth the extra cognitive load to extend nosplit throughout all of the locking functions. (2) is a real issue which would cause incorrect ordering, but it is already handled by switching to the system stack before recording the lock ordering. Adding / removing nosplit has no effect on this issue. Change-Id: I94fbd21b2bf928dbf1bf71aabb6788fc0a012829 Reviewed-on: https://go-review.googlesource.com/c/go/+/254367 Run-TryBot: Michael Pratt <mpratt@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Dan Scales <danscales@google.com> Trust: Michael Pratt <mpratt@google.com> |
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