multithreading broken in Cygwin 3.5.3

[Bruno Haible]

Hi,

In Cygwin 3.5.3, on different machines, I see 3 Gnulib tests failing by
timeout that worked perfectly fine in Cygwin 3.4.6 and older:
  FAIL: test-call_once2.exe
  FAIL: test-lock.exe
  FAIL: test-pthread-once2.exe

Find here attached a simplified version of test-pthread-once2.c.
Compile and run:
  $ x86_64-pc-cygwin-gcc -Wall foo.c
  $ ./a

Expected behaviour: Termination within 1 minute.
Actual behaviour:   Terminates by timeout after 10 minutes.

When I change
  #define ENABLE_DEBUGGING 0
to
  #define ENABLE_DEBUGGING 1
the test does lots of output and terminates within 20 seconds.
Therefore I can't really tell where the problem comes from.
But I do see some changes in
  $ git diff cygwin-3.4.6 cygwin-3.5.3 winsup/cygwin/thread.cc
  $ git diff cygwin-3.4.6 cygwin-3.5.3 winsup/testsuite/winsup.api/pthread

Bruno
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