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Date: Fri, 9 Apr 2021 15:11:14 +0300
From: "Andrey Bugaevskiy" <bugaevskiy@...dex-team.ru>
To: "'Rich Felker'" <dalias@...c.org>
Cc: "'Florian Weimer'" <fweimer@...hat.com>,
<musl@...ts.openwall.com>
Subject: RE: errno and swapcontext in a multithreaded setup
On Thu, 8 Apr 2021, Rich Felker wrote:
> On Thu, Apr 08, 2021 at 08:46:00PM +0300, Andrey Bugaevskiy wrote:
> > On Thu, 8 Apr 2021, Florian Weimer wrote:
> > > * Andrey Bugaevskiy:
> > >
> > > > I'm using makecontext/swapcontext to migrate contexts between
threads
> > > > and this sometimes leads to getting incorrect errno values.
> > > >
> > > > Investigating further I've noticed that __errno_location
> > > > is marked __attribute__((const)).
> > > > This causes optimizers to assume that errno address never changes
> > > > in the scope of the function which is not the case in my scenario.
> > >
> > > The optimizers make the same assumption for actual thread-local
> > > variables, not just __errno_location. Migrating contexts between
> > > threads results in undefined behavior.
> >
> > Can you please point me to some explanation why this optimization
> > is valid for thread-local variables?
> >
> > As far as I can imagine optimizer should at least prove that it
> > won't be changed from some other place or (if the variable is local
> > to the function) that it is not changed by a recursive call.
>
> Perhaps you're confusing the value of errno with its address. No
> function can change the address of errno, only its value.
> Conceptually, __errno_location() is just &errno.
Well, it's a function returning a value, but I see the point.
I'm still a bit confused about why optimizer can assume that
TLS location never changes though.
Thanks to everyone for their replies.
--
Andrey Bugaevskiy
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