Fix write hang bug found by syzkaller.

After this change e.mu is only promoted to exclusively locked during
route.Resolve. It downgrades back to read-lock afterwards.
This prevents the second RLock() call gets stuck later in the stack.

https://syzkaller.appspot.com/bug?id=065b893bd8d1d04a4e0a1d53c578537cde1efe99

Syzkaller logs does not contain interesting stack traces.
The following stack trace is obtained by running repro locally.

goroutine 53 [semacquire, 3 minutes]:
runtime.gopark(0xfd4278, 0x1896320, 0xc000301912, 0x4)
        GOROOT/src/runtime/proc.go:304 +0xe0 fp=0xc0000e25f8 sp=0xc0000e25d8 pc=0x437170
runtime.goparkunlock(...)
        GOROOT/src/runtime/proc.go:310
runtime.semacquire1(0xc0001220b0, 0xc00000a300, 0x1, 0x0)
        GOROOT/src/runtime/sema.go:144 +0x1c0 fp=0xc0000e2660 sp=0xc0000e25f8 pc=0x4484e0
sync.runtime_Semacquire(0xc0001220b0)
        GOROOT/src/runtime/sema.go:56 +0x42 fp=0xc0000e2690 sp=0xc0000e2660 pc=0x448132
gvisor.dev/gvisor/pkg/sync.(*RWMutex).RLock(...)
        pkg/sync/rwmutex_unsafe.go:76
gvisor.dev/gvisor/pkg/tcpip/transport/udp.(*endpoint).HandleControlPacket(0xc000122000, 0x7ee5, 0xc00053c16c, 0x4, 0x5e21, 0xc00053c224, 0x4, 0x1, 0x0, 0xc00007ed00)
        pkg/tcpip/transport/udp/endpoint.go:1345 +0x169 fp=0xc0000e26d8 sp=0xc0000e2690 pc=0x9843f9
......
gvisor.dev/gvisor/pkg/tcpip/transport/udp.(*protocol).HandleUnknownDestinationPacket(0x18bb5a0, 0xc000556540, 0x5e21, 0xc00053c16c, 0x4, 0x7ee5, 0xc00053c1ec, 0x4, 0xc00007e680, 0x4)
        pkg/tcpip/transport/udp/protocol.go:143 +0xb9a fp=0xc0000e8260 sp=0xc0000e7510 pc=0x9859ba
......
gvisor.dev/gvisor/pkg/tcpip/transport/udp.sendUDP(0xc0001220d0, 0xc00053ece0, 0x1, 0x1, 0x883, 0x1405e217ee5, 0x11100a0, 0xc000592000, 0xf88780)
        pkg/tcpip/transport/udp/endpoint.go:924 +0x3b0 fp=0xc0000ed390 sp=0xc0000ec750 pc=0x981af0
gvisor.dev/gvisor/pkg/tcpip/transport/udp.(*endpoint).write(0xc000122000, 0x11104e0, 0xc00020a460, 0x0, 0x0, 0x0, 0x0, 0x0)
        pkg/tcpip/transport/udp/endpoint.go:510 +0x4ad fp=0xc0000ed658 sp=0xc0000ed390 pc=0x97f2dd

PiperOrigin-RevId: 315590041

0 files changed, 0 insertions, 0 deletions