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
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.
Run-TryBot: Michael Pratt <firstname.lastname@example.org>
Reviewed-by: Austin Clements <email@example.com>
TryBot-Result: Gobot Gobot <firstname.lastname@example.org>
(cherry picked from commit 11b3730)
Reviewed-by: Cherry Zhang <email@example.com>