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Message-ID: <CAHmME9qDzZV_JQbTE4vsWiRsWeF3tqyhNqBLisY3_xPnMviqHQ@mail.gmail.com>
Date: Wed, 16 Sep 2026 17:23:00 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Rich Felker <dalias@...c.org>
Cc: Demi Marie Obenour <demiobenour@...il.com>, musl@...ts.openwall.com,
Jonas Böttiger <jonasboettiger@...oud.com>,
AWilcox@...cox-tech.com
Subject: Re: vDSO-based getrandom
On Sun, Aug 23, 2026 at 11:44 PM Rich Felker <dalias@...c.org> wrote:
>
> On Sun, Aug 23, 2026 at 03:12:39PM -0600, Jason A. Donenfeld wrote:
> > On Sun, Aug 23, 2026, 13:53 Rich Felker <dalias@...c.org> wrote:
> > > On Sun, Aug 23, 2026 at 03:03:43PM -0400, Demi Marie Obenour wrote:
> > > > I agree that this would be simpler, but I’m concerned that it could
> > > > (a) be racy and (b) be a scalability bottleneck. I suspect the vDSO
> > > > doesn’t guarantee that the atomics used have strong enough memory
> > > > barriers for multithreaded use. Furthermore, modern server systems
> > > > can have a very large number of cores and a global lock could be a
> > > > serious scalability bottleneck.
> > >
> > > The strength of the atomics is a good question. If the ones used
> > > internally are not sufficient you could do the same thing around it
> > > with full strength atomics.
> > >
> > > I don't think allocating an extra page of memory per thread is very
> > > nice.
> > >
> >
> > It's not a page per thread. You allocate a page (or pages) and divide it
> > between threads.
>
> Thanks for clarifying. So I guess you need to maintain some sort of
> allocator state for which slots are used/free? Or one could try to
> make it work with selecting a slot based on getcpu instead of
> statically assigning them to threads.
Yea. There's some demo code that's a lot simpler than the glibc stuff,
that you could adapt, or at least read to get a more straight forward
idea of how this is supposed to work.
(Sorry for the delay; I've been traveling.)
Voila: https://git.zx2c4.com/linux-rng/tree/tools/testing/selftests/vDSO/vdso_test_getrandom.c
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