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

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Message-ID: <87bkdmznl6.fsf@oldenburg.str.redhat.com>
Date:   Thu, 28 Sep 2023 10:52:37 +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:

> On Tue, 26 Sept 2023 at 21:51, Florian Weimer <fweimer@...hat.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