Date: Thu, 15 Oct 2020 13:13:16 -0400 From: Rich Felker <dalias@...c.org> To: Alexey Izbyshev <izbyshev@...ras.ru> Cc: musl@...ts.openwall.com Subject: Re: Why is setrlimit() considered to have per-thread effect? 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. 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 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actual (code) content though rather than just as a mention in commit messages, since it's contrary to what my (probably outdated) understanding of how rlimits worked was. >  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c022a0acad534fd5f5d5f17280f6d4d135e74e81 >  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
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