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cmd/internal/obj, runtime: use register map to mark unsafe points 

Currently we use stack map index -2 to mark unsafe points, i.e.
PC ranges that is not safe for async preemption. This has a
problem: it cannot mark CALL instructions, because for stack scan
a valid stack map index is needed.

This CL switches to use register map index for marking unsafe
points instead, which does not conflict with stack scan and can
be applied on CALL instructions. This is necessary as next CL
will mark call to morestack nonpreemptible.

For #35470.

Change-Id: I357bf26c996e1fee1e7eebe4e6bb07d62930d3f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/207349
Reviewed-by: David Chase <drchase@google.com>
This commit is contained in:
Cherry Zhang 2019-11-14 17:03:44 -05:00
parent 4a378d712d
commit c3f149250e
2 changed files with 4 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
src/cmd/internal/obj/plist.go
View File

@ -215,12 +215,10 @@ func (ctxt *Link) StartUnsafePoint(p *Prog, newprog ProgAlloc) *Prog {
pcdata := Appendp(p, newprog)
pcdata.As = APCDATA
pcdata.From.Type = TYPE_CONST
pcdata.From.Offset = objabi.PCDATA_StackMapIndex
pcdata.From.Offset = objabi.PCDATA_RegMapIndex
pcdata.To.Type = TYPE_CONST
pcdata.To.Offset = -2 // pcdata -2 marks unsafe point
// TODO: register map?
return pcdata
}
@ -232,7 +230,7 @@ func (ctxt *Link) EndUnsafePoint(p *Prog, newprog ProgAlloc, oldval int64) *Prog
pcdata := Appendp(p, newprog)
pcdata.As = APCDATA
pcdata.From.Type = TYPE_CONST
pcdata.From.Offset = objabi.PCDATA_StackMapIndex
pcdata.From.Offset = objabi.PCDATA_RegMapIndex
pcdata.To.Type = TYPE_CONST
pcdata.To.Offset = oldval
@ -248,7 +246,7 @@ func MarkUnsafePoints(ctxt *Link, p0 *Prog, newprog ProgAlloc, isUnsafePoint fun
prev := p0
oldval := int64(-1) // entry pcdata
for p := prev.Link; p != nil; p, prev = p.Link, p {
if p.As == APCDATA && p.From.Offset == objabi.PCDATA_StackMapIndex {
if p.As == APCDATA && p.From.Offset == objabi.PCDATA_RegMapIndex {
oldval = p.To.Offset
continue
}

2
src/runtime/preempt.go
View File

@ -393,7 +393,7 @@ func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) bool {
// use the LR for unwinding, which will be bad.
return false
}
smi := pcdatavalue(f, _PCDATA_StackMapIndex, pc, nil)
smi := pcdatavalue(f, _PCDATA_RegMapIndex, pc, nil)
if smi == -2 {
// Unsafe-point marked by compiler. This includes
// atomic sequences (e.g., write barrier) and nosplit