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runtime: fix generation skew with trace reentrancy

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Currently when performing multiple nested traceAcquires, we re-read
trace.gen on subsequent reads. But this is invalid, since a generation
transition may happen in between a traceAcquire and a nested
traceAcquire. The first one will produce a traceLocker with a gen from
the previous generation, and the second will produce a traceLocker from
the next generation. (Note: generations cannot _complete_ advancement
under traceAcquire, but trace.gen can move forward.) The end result is
earlier events, from the nested traceAcquire, will write to a future
generation, and then previous events will write to a past generation.
This can break the trace.

(There are also a lot of comments left over talking about the
non-reentrancy of the tracer; we should look at those again.)

Change-Id: I08ac8cc86d41ab3e6061c5de58d657b6ad0d19d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/708397
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
This commit is contained in:
Michael Anthony Knyszek 2025-09-30 23:54:07 +00:00 • committed by Michael Knyszek
parent df33c17091
commit dc9a3e2a65
1 changed files with 3 additions and 1 deletions
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4
src/runtime/traceruntime.go
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@ -29,6 +29,7 @@ type mTraceState struct {
buf [2][tracev2.NumExperiments]*traceBuf // Per-M traceBuf for writing. Indexed by trace.gen%2. buf [2][tracev2.NumExperiments]*traceBuf // Per-M traceBuf for writing. Indexed by trace.gen%2.
link *m // Snapshot of alllink or freelink. link *m // Snapshot of alllink or freelink.
reentered uint32 // Whether we've reentered tracing from within tracing. reentered uint32 // Whether we've reentered tracing from within tracing.
entryGen uintptr // The generation value on first entry.
oldthrowsplit bool // gp.throwsplit upon calling traceLocker.writer. For debugging. oldthrowsplit bool // gp.throwsplit upon calling traceLocker.writer. For debugging.
} }
@ -212,7 +213,7 @@ func traceAcquireEnabled() traceLocker {
// that it is. // that it is.
if mp.trace.seqlock.Load()%2 == 1 { if mp.trace.seqlock.Load()%2 == 1 {
mp.trace.reentered++ mp.trace.reentered++
return traceLocker{mp, trace.gen.Load()} return traceLocker{mp, mp.trace.entryGen}
} }
// Acquire the trace seqlock. This prevents traceAdvance from moving forward // Acquire the trace seqlock. This prevents traceAdvance from moving forward
@ -240,6 +241,7 @@ func traceAcquireEnabled() traceLocker {
releasem(mp) releasem(mp)
return traceLocker{} return traceLocker{}
} }
mp.trace.entryGen = gen
return traceLocker{mp, gen} return traceLocker{mp, gen}
} }