[Bug nptl/30041] pthread_cancel() hangs on aarch64

["stsp at users dot sourceforge.net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30041

--- Comment #8 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Andreas Schwab from comment #7)
> If anything this is a bug in the debugger.  There are two concurrent types
> of events, the signal and the shlib events, and the constant flow of signal
> events prevents the shlib event from making forward progress.

Yes, this seems to be the case.
I modified the test so that the
second thread disables the timer
after some time. If pthread_cancel()
was hanging, it unstucks.
If pthread_join() was hanging, it
doesn't unstuck because actually
the second thread is already terminated
so the timer shut-down doesn't happen.

But I thought I excluded such a possibility
by at least 2 things:
- attaching with gdb and doing "handle SIGALRM nopass"
- lowering the SIGALRM rate and making
sure both threads can execute code and
print things.

So I still don't understand what's
going on. If both threads could
sleep() and printf() relatively happily
under the much higher SIGALRM rate, then
why some rather small SIGALRM rate still
causes pthread_cancel() to stall indefinitely?
Its not like anything else stalls.
In fact, I discovered that effect on a
real program of mine, which works perfectly
(and is used by people) under the exact
SIGALRM rate which causes the full stall of
pthread_cancel().
So how is that possible w/o a bug?

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