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From: Florian Weimer <fweimer@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: =?utf-8?Q?Andr=C3=A9?= Almeida <andrealmeid@igalia.com>,  Mathieu
 Desnoyers
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Subject: Re: [RFC PATCH 0/1] Add FUTEX_SPIN operation
In-Reply-To: <20240502-gezeichnet-besonderen-d277879cd669@brauner> (Christian
	Brauner's message of "Thu, 2 May 2024 10:45:41 +0200")
References: <20240425204332.221162-1-andrealmeid@igalia.com>
	<20240426-gaumen-zweibeinig-3490b06e86c2@brauner>
	<f052ff72-72c9-4b83-9285-2cd9d52e5f72@igalia.com>
	<20240502-gezeichnet-besonderen-d277879cd669@brauner>
Date: Thu, 02 May 2024 11:51:56 +0200
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* Christian Brauner:

> Unless I'm missing something the question here is PID (as in TGID aka
> thread-group leader id gotten via getpid()) vs TID (thread specific id
> gotten via gettid()). You want the thread-specific id as you want to
> interact with the futex state of a specific thread not the thread-group
> leader.
>
> Aside from that TIDs are subject to the same race conditions that PIDs
> are. They are allocated from the same pool (see alloc_pid()).

For most mutex types (but not robust mutexes), it is undefined in
userspace if a thread exits while it has locked a mutex.  Such a usage
condition would ensure that the race doesn't happen, I believe.

From a glibc perspective, we typically cannot use long-term file
descriptors (that are kept open across function calls) because some
applications do not expect them, or even close them behind our back.

Thanks,
Florian