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Message-ID: <20260823214507.GN23438@brightrain.aerifal.cx>
Date: Sun, 23 Aug 2026 17:45:07 -0400
From: Rich Felker <dalias@...c.org>
To: "Jason A. Donenfeld" <Jason@...c4.com>
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 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.
Rich
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