Re: [RFC PATCH 0/2] futex: how to solve the robust_list race condition?

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-03-02 10:32, Florian Weimer wrote:
> * Mathieu Desnoyers:
> 
>> On 2026-03-02 02:31, Florian Weimer wrote:
>>> * Mathieu Desnoyers:
>>>
>>>> Of course, we'd have to implement the whole transaction in assembler
>>>> for each architecture.
>>> Could this be hidden ina vDSO call?
>>
[...]
>> I suspect the IP ranges and associated store-conditional flags I identified
>> for the rseq_rl_cs approach are pretty much the key states we need to track.
>> Architectures which support atomic exchange instructions are even simpler.
>> We'd just have to keep track of this unlock operations steps internally
>> between the kernel and the vDSO.
> 
> If the unlock operation is in the vDSO, we need to parameterize it
> somehow, regarding offsets, values written etc., so that it's not
> specific to exactly one robust mutex implementation.

Agreed.

> 
>> But you mentioned that rseq would be needed for a flag, so what I am
>> missing ?
> 
> It's so that you don't have to figure out that the program counter is
> somewhere in the special robust mutex unlock code every time a task gets
> descheduled.

AFAIU we don't need to evaluate this on context switch. We only need
to evaluate it at:

(a) Signal delivery,
(b) Process exit.

Also, the tradeoff here is not clear cut to me: the only thing the rseq
flag would prevent is comparisons of the instruction pointer against a
vDSO range at (a) and (b), which are not as performance critical as
context switches. I'm not sure it would warrant the added complexity of
the rseq flag, and coupling with rseq. Moreover, I'm not convinced that
loading an extra rseq flag field from userspace would be faster than
just comparing with a known range of vDSO addresses.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com