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rsc opened this issue Aug 7, 2015 · 5 comments
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runtime: missed write barrier with race detector #12068

rsc opened this issue Aug 7, 2015 · 5 comments
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Go1.5

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@rsc
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rsc commented Aug 7, 2015

Reported on golang-dev by carl@stripe.com. This program, reduced from a much larger one, crashes due to GC missing objects when run under the race detector. Best suspect is missed write barrier in that mode.

GODEBUG=gccheckmark=1 go run -race heavy.go 

package main

type Interface interface {
    Dummy()
}

type Thing1 struct {
    a, b *int
}

type Thing2 struct {
    Thing1
}

func (_ Thing2) Dummy() {}

func NewThing1() Thing1 {
    return Thing1{new(int), new(int)}
}

func NewThing2() Interface {
    return &Thing2{NewThing1()}
}

func main() {
    for i := 0; i < 20; i++ {
        go func() {
            for {
                t := NewThing2()
                t.Dummy()
            }
        }()
    }
    <-make(chan struct{})
}

Damian Gryski suggests perhaps the bug was introduced on github.com/golang/go/commit/8f34b25318e712a18c3847bb1cc3e8d87076c211.
@rsc rsc self-assigned this Aug 7, 2015
@rsc rsc added this to the Go1.5 milestone Aug 7, 2015
@rsc
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rsc commented Aug 7, 2015

@aclements @RLH FYI, but I don't think this is the GC's fault.

@dgryski
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dgryski commented Aug 7, 2015

A more accurate description of my git-bisect attempts is that there have been a number of commits that made this occur with increasing frequency. Ranging from once every 20min of testing at the above commit to every 3-4 seconds with tip. These changes tend to be at a point where the gc has been tweaked.

@rsc rsc changed the title cmd/compile: missed write barrier with race detector runtime: missed write barrier with race detector Aug 7, 2015
@rsc
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rsc commented Aug 7, 2015

Variant that exits, for testing:

package main

type Interface interface {
    Dummy()
}

type Thing1 struct {
    a, b *int
}

type Thing2 struct {
    Thing1
}

func (_ Thing2) Dummy() {}

func NewThing1() Thing1 {
    return Thing1{new(int), new(int)}
}

func NewThing2() Interface {
    return &Thing2{NewThing1()}
}

func hammer(n int, c chan int) {
    for i := 0; i < n; i++ {
        t := NewThing2()
        t.Dummy()
    }
    c <- 1
}

func main() {
    c := make(chan int)
    for i := 0; i < 20; i++ {
        go hammer(2000000, c)
    }
    for i := 0; i < 20; i++ {
        <-c
    }
}

@rsc
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rsc commented Aug 7, 2015

Fix in CL 13364.

@rsc rsc closed this as completed in 3ae1704 Aug 7, 2015
@zenazn
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zenazn commented Aug 7, 2015

Thanks, @rsc!

@adg adg mentioned this issue Aug 10, 2015
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@golang golang locked and limited conversation to collaborators Aug 8, 2016
@rsc rsc removed their assignment Jun 23, 2022
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