Re: [RFC PATCH v2 1/4] rseq: Add sched_state field to struct rseq

[Florian Weimer]

* Dmitry Vyukov:

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