[Bug libc/30558] SIGEV_THREAD is badly implemented

["bugdal at aerifal dot cx" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30558

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
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                 CC|                            |bugdal at aerifal dot cx

--- Comment #28 from Rich Felker <bugdal at aerifal dot cx> ---
FWIW, I think I do read the specification as requiring a "new thread" for the
timer expiration event, but this does not conflict with an implementation
choice to reuse the memory and kernel task from a previous thread *after its
lifetime has ended* to provide the "new thread". This is what we do in musl.

This does of course preclude delivering a second timer event while the previous
one is still running (yielding overruns). I view that a as a very positive QoI
property since it prevents cascading resources usage leading to exhaustion and
failure under load, and lets the application handle the overruns in some
reasonable way instead. But if you did want to allow concurrent expiration
functions on the same timer, the timer thread could attempt to first create a
real new thread, and only fallback to running the function in its own context
when there are insufficient resources to make a new thread or some
implementation-chosen concurrency limit is reached.

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