linux-kernel - Re: [RFC PATCH v2 1/4] rseq: Add sched

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Message-ID: <875y3wp6au.fsf@oldenburg.str.redhat.com>
Date:   Wed, 27 Sep 2023 06:51:37 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     mathieu.desnoyers@...icios.com, David.Laight@...lab.com,
        alexander@...alicyn.com, andrealmeid@...lia.com,
        boqun.feng@...il.com, brauner@...nel.org, carlos@...hat.com,
        ckennelly@...gle.com, corbet@....net, dave@...olabs.net,
        dvhart@...radead.org, goldstein.w.n@...il.com, hpa@...or.com,
        libc-alpha@...rceware.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, longman@...hat.com, mingo@...hat.com,
        paulmck@...nel.org, peterz@...radead.org, pjt@...gle.com,
        posk@...k.io, rostedt@...dmis.org, tglx@...utronix.de
Subject: Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

* 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