go122-create-syscall-reuse-thread-id.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Tests a G being created from within a syscall.
     6  //
     7  // Specifically, it tests a scenario wherein a C
     8  // thread is calling into Go, creating a goroutine in
     9  // a syscall (in the tracer's model). The system is free
    10  // to reuse thread IDs, so first a thread ID is used to
    11  // call into Go, and then is used for a Go-created thread.
    12  //
    13  // This is a regression test. The trace parser didn't correctly
    14  // model GoDestroySyscall as dropping its P (even if the runtime
    15  // did). It turns out this is actually fine if all the threads
    16  // in the trace have unique IDs, since the P just stays associated
    17  // with an eternally dead thread, and it's stolen by some other
    18  // thread later. But if thread IDs are reused, then the tracer
    19  // gets confused when trying to advance events on the new thread.
    20  // The now-dead thread which exited on a GoDestroySyscall still has
    21  // its P associated and this transfers to the newly-live thread
    22  // in the parser's state because they share a thread ID.
    23  
    24  package main
    25  
    26  import (
    27  	"internal/trace"
    28  	"internal/trace/internal/testgen"
    29  	"internal/trace/tracev2"
    30  	"internal/trace/version"
    31  )
    32  
    33  func main() {
    34  	testgen.Main(version.Go122, gen)
    35  }
    36  
    37  func gen(t *testgen.Trace) {
    38  	g := t.Generation(1)
    39  
    40  	// A C thread calls into Go and acquires a P. It returns
    41  	// back to C, destroying the G.
    42  	b0 := g.Batch(trace.ThreadID(0), 0)
    43  	b0.Event("GoCreateSyscall", trace.GoID(4))
    44  	b0.Event("GoSyscallEndBlocked")
    45  	b0.Event("ProcStatus", trace.ProcID(0), tracev2.ProcIdle)
    46  	b0.Event("ProcStart", trace.ProcID(0), testgen.Seq(1))
    47  	b0.Event("GoStatus", trace.GoID(4), trace.NoThread, tracev2.GoRunnable)
    48  	b0.Event("GoStart", trace.GoID(4), testgen.Seq(1))
    49  	b0.Event("GoSyscallBegin", testgen.Seq(2), testgen.NoStack)
    50  	b0.Event("GoDestroySyscall")
    51  
    52  	// A new Go-created thread with the same ID appears and
    53  	// starts running, then tries to steal the P from the
    54  	// first thread. The stealing is interesting because if
    55  	// the parser handles GoDestroySyscall wrong, then we
    56  	// have a self-steal here potentially that doesn't make
    57  	// sense.
    58  	b1 := g.Batch(trace.ThreadID(0), 0)
    59  	b1.Event("ProcStatus", trace.ProcID(1), tracev2.ProcIdle)
    60  	b1.Event("ProcStart", trace.ProcID(1), testgen.Seq(1))
    61  	b1.Event("ProcSteal", trace.ProcID(0), testgen.Seq(3), trace.ThreadID(0))
    62  }
    63
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