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runtime: fix race in scanvalid assertion

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Change-Id: I389b2e10fe667eaa55f87b71b1e004994694d4a3
Reviewed-on: https://go-review.googlesource.com/11173
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Russ Cox 2015-06-17 13:56:03 -04:00
parent f0fee976aa
commit cfa3eda587
1 changed files with 21 additions and 4 deletions
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25
src/runtime/proc1.go
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@ -248,6 +248,18 @@ func readgstatus(gp *g) uint32 {
return atomicload(&gp.atomicstatus) return atomicload(&gp.atomicstatus)
} }
// Ownership of gscanvalid:
//
// If gp is running (meaning status == _Grunning or _Grunning|_Gscan),
// then gp owns gp.gscanvalid, and other goroutines must not modify it.
//
// Otherwise, a second goroutine can lock the scan state by setting _Gscan
// in the status bit and then modify gscanvalid, and then unlock the scan state.
//
// Note that the first condition implies an exception to the second:
// if a second goroutine changes gp's status to _Grunning|_Gscan,
// that second goroutine still does not have the right to modify gscanvalid.
// The Gscanstatuses are acting like locks and this releases them. // The Gscanstatuses are acting like locks and this releases them.
// If it proves to be a performance hit we should be able to make these // If it proves to be a performance hit we should be able to make these
// simple atomic stores but for now we are going to throw if // simple atomic stores but for now we are going to throw if
@ -294,10 +306,6 @@ func castogscanstatus(gp *g, oldval, newval uint32) bool {
return cas(&gp.atomicstatus, oldval, newval) return cas(&gp.atomicstatus, oldval, newval)
} }
case _Grunning: case _Grunning:
if gp.gcscanvalid {
print("runtime: castogscanstatus _Grunning and gp.gcscanvalid is true, newval=", hex(newval), "\n")
throw("castogscanstatus")
}
if newval == _Gscanrunning || newval == _Gscanenqueue { if newval == _Gscanrunning || newval == _Gscanenqueue {
return cas(&gp.atomicstatus, oldval, newval) return cas(&gp.atomicstatus, oldval, newval)
} }
@ -320,6 +328,15 @@ func casgstatus(gp *g, oldval, newval uint32) {
}) })
} }
if oldval == _Grunning && gp.gcscanvalid {
// If oldvall == _Grunning, then the actual status must be
// _Grunning or _Grunning|_Gscan; either way,
// we own gp.gcscanvalid, so it's safe to read.
// gp.gcscanvalid must not be true when we are running.
print("runtime: casgstatus ", hex(oldval), "->", hex(newval), " gp.status=", hex(gp.atomicstatus), " gp.gcscanvalid=true\n")
throw("casgstatus")
}
// loop if gp->atomicstatus is in a scan state giving // loop if gp->atomicstatus is in a scan state giving
// GC time to finish and change the state to oldval. // GC time to finish and change the state to oldval.
for !cas(&gp.atomicstatus, oldval, newval) { for !cas(&gp.atomicstatus, oldval, newval) {