Re: [musl] Why is setrlimit() considered to have per-thread effect?

[Rich Felker <dalias@libc.org>]

On Thu, Oct 15, 2020 at 07:13:30PM +0300, Alexey Izbyshev wrote:
> On 2020-10-15 18:49, Rich Felker wrote:
> >setrlimit implemented in terms of prlimit does; as far as I can tell
> >prlimit does not perform any process-global action itself but just
> >lets you target different tasks. This means we *could* "optimize"
> >setrlimit to skip __synccall and instead just iterate over the thread
> >list and SYS_prlimit each one from the calling thread context.
> >
> >The prlimit function on the other hand behaves as the Linux syscall
> >and lets you set thread-specific limits.
> >
> But in my understanding, prlimit() sets process- (not thread-)
> specific limits, and have done so since its introduction[1]. The

That was not my understanding, but it may be true. I would not assume
it's true just because of the word "process" in a commit message or
comment since kernel folks, especially in that era, regularly used
"process" and "thread" interchangibly/inconsistently.

> code operates on "signal" structure which is shared between threads
> of a thread group. Further, an earlier commit[2] explicitly says
> that "...rlimit are
> per process and not per-thread.". It's true that in pre-2.6.10
> kernels setrlimit() operated in per-thread limits (see my reply to
> Szabolcs), but it's not related to prlimit() syscall, which was
> added much later.
> 
> To be clear, I did not propose to optimize setrlimit() in my initial
> email, I was just surprised that synccall() is needed at all. But if
> we want optimization, it seems that trying prlimit() first and
> falling back to synccall() in case of ENOSYS would be what we want.

If correct, I agree -- we can avoid the need for __synccall when
prlimit works. I'd like to find commits or source lines supporting
that in their actual (code) content though rather than just as a
mention in commit messages, since it's contrary to what my (probably
outdated) understanding of how rlimits worked was.

> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c022a0acad534fd5f5d5f17280f6d4d135e74e81
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7855c35da7ba16b389d17710401c4a55a3ea2102

Somewhat off-topic, but for some reason that second link is bringing
my browser to a crawl swapping, despite the commit being tiny when I
view it locally in my kernel tree. Weird.

Rich