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runtime: preemption in startTemplateThread may cause infinite hang [1.14 backport] #38933

gopherbot opened this issue May 7, 2020 · 2 comments
CherryPickApproved FrozenDueToAge


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@gopherbot gopherbot commented May 7, 2020

@prattmic requested issue #38931 to be considered for backport to the next 1.14 minor release.

@gopherbot backport please 1.13 1.14

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@gopherbot gopherbot commented May 21, 2020

Change mentions this issue: [release-branch.go1.14] runtime: disable preemption in startTemplateThread

@andybons andybons added the CherryPickApproved label May 27, 2020
@gopherbot gopherbot removed the CherryPickCandidate label May 27, 2020
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@gopherbot gopherbot commented May 27, 2020

Closed by merging 5112209 to release-branch.go1.14.

gopherbot pushed a commit that referenced this issue May 27, 2020

When a locked M wants to start a new M, it hands off to the template
thread to actually call clone and start the thread. The template thread
is lazily created the first time a thread is locked (or if cgo is in

stoplockedm will release the P (_Pidle), then call handoffp to give the
P to another M. In the case of a pending STW, one of two things can

1. handoffp starts an M, which does acquirep followed by schedule, which
will finally enter _Pgcstop.

2. handoffp immediately enters _Pgcstop. This only occurs if the P has
no local work, GC work, and no spinning M is required.

If handoffp starts an M, and must create a new M to do so, then newm
will simply queue the M on newmHandoff for the template thread to do the

When a stop-the-world is required, stopTheWorldWithSema will start the
stop and then wait for all Ps to enter _Pgcstop. If the template thread
is not fully created because startTemplateThread gets stopped, then
another stoplockedm may queue an M that will never get created, and the
handoff P will never leave _Pidle. Thus stopTheWorldWithSema will wait

A sequence to trigger this hang when STW occurs can be visualized with
two threads:

  T1                                 T2
-------------------------------   -----------------------------

LockOSThread                      LockOSThread
  haveTemplateThread == 0
    haveTemplateThread = 1
    newm                            haveTemplateThread == 1
      preempt -> schedule           g.m.lockedExt++
        gcstopm -> _Pgcstop         g.m.lockedg = ...
        park                        g.lockedm = ...

                                 ... (any code)
                                   preempt -> schedule
                                       releasep -> _Pidle
                                         startm (first 3 handoffp cases)
                                            g.m.lockedExt != 0
                                            Add to newmHandoff, return

Note that the P in T2 is stuck sitting in _Pidle. Since the template
thread isn't running, the new M will not be started complete the
transition to _Pgcstop.

To resolve this, we disable preemption around the assignment of
haveTemplateThread and the creation of the template thread in order to
guarantee that if handTemplateThread is set then the template thread
will eventually exist, in the presence of stops.

For #38931
Fixes #38933

Change-Id: I50535fbbe2f328f47b18e24d9030136719274191
Run-TryBot: Michael Pratt <>
Reviewed-by: Austin Clements <>
TryBot-Result: Gobot Gobot <>
(cherry picked from commit 11b3730)
Reviewed-by: Cherry Zhang <>
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