Commits on Nov 12, 2024

1. cygthread: suspend thread before terminating. …

   This addresses an extremely difficult to debug deadlock when running
   under emulation on ARM64.
   
   A relatively easy way to trigger this bug is to call `fork()`, then within the
   child process immediately call another `fork()` and then `exit()` the
   intermediate process.
   
   It would seem that there is a "code emulation" lock on the wait thread at
   this stage, and if the thread is terminated too early, that lock still exists
   albeit without a thread, and nothing moves forward.
   
   It seems that a `SuspendThread()` combined with a `GetThreadContext()`
   (to force the thread to _actually_ be suspended, for more details see
   https://devblogs.microsoft.com/oldnewthing/20150205-00/?p=44743)
   makes sure the thread is "booted" from emulation before it is suspended.
   
   Hopefully this means it won't be holding any locks or otherwise leave
   emulation in a bad state when the thread is terminated.
   
   Also, attempt to use `CancelSynchonousIo()` (as seen in `flock.cc`) to avoid
   the need for `TerminateThread()` altogether.  This doesn't always work,
   however, so was not a complete fix for the deadlock issue.
   
   Addresses: https://cygwin.com/pipermail/cygwin-developers/2024-May/012694.html
   Signed-off-by: Jeremy Drake <[email protected]>

   

   jeremyd2019 committed Nov 12, 2024
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