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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>
        <875y3wp6au.fsf@oldenburg.str.redhat.com>
        <CACT4Y+aVY+KPmBXYDBpA+JmLHvk=o5bt9d4tV5L41e813e0SVw@mail.gmail.com>
Date:   Thu, 28 Sep 2023 10:52:37 +0200
In-Reply-To: <CACT4Y+aVY+KPmBXYDBpA+JmLHvk=o5bt9d4tV5L41e813e0SVw@mail.gmail.com>
        (Dmitry Vyukov's message of "Wed, 27 Sep 2023 08:58:16 -0700")
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* Dmitry Vyukov:

> On Tue, 26 Sept 2023 at 21:51, Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * 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.
>
> I agree. I wouldn't do this if there were other options. That's why I
> am looking for official kernel support for this.
> If we would have a separate 8 bytes that are overwritten with 0 when a
> thread is descheduled, that would be perfect.

That only solves part of the problem because these fields would still
have to be locked to tcmalloc.  I think you'd need a rescheduling
counter, then every library could keep their reference values in
library-private thread-local storage.

Thanks,
Florian