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From:   Florian Weimer <fweimer@redhat.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     mathieu.desnoyers@efficios.com, David.Laight@aculab.com,
        alexander@mihalicyn.com, andrealmeid@igalia.com,
        boqun.feng@gmail.com, brauner@kernel.org, carlos@redhat.com,
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        posk@posk.io, rostedt@goodmis.org, tglx@linutronix.de
Subject: Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq
References: <2c421e36-a749-7dc3-3562-7a8cf256df3c@efficios.com>
        <20230926205215.472650-1-dvyukov@google.com>
        <CACT4Y+beLh1qnHF9bxhMUcva8KyuvZs7Mg_31SGK5xSoR=3m1A@mail.gmail.com>
Date:   Wed, 27 Sep 2023 06:51:37 +0200
In-Reply-To: <CACT4Y+beLh1qnHF9bxhMUcva8KyuvZs7Mg_31SGK5xSoR=3m1A@mail.gmail.com>
        (Dmitry Vyukov's message of "Tue, 26 Sep 2023 16:49:32 -0700")
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* Dmitry Vyukov:

> In reality it's a bit more involved since the field is actually 8
> bytes and only partially overlaps with rseq.cpu_id_start (it's an
> 8-byte pointer with high 4 bytes overlap rseq.cpu_id_start):
>
> https://github.com/google/tcmalloc/blob/229908285e216cca8b844c1781bf16b838128d1b/tcmalloc/internal/percpu.h#L101-L165

This does not compose with other rseq users, as noted in the sources:

  // Note: this makes __rseq_abi.cpu_id_start unusable for its original purpose.

For a core library such a malloc replacement, that is a very bad trap.

Thanks,
Florian