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Date: Thu, 8 Apr 2021 14:55:11 -0400
From: Rich Felker <dalias@libc.org>
To: Andrey Bugaevskiy <bugaevskiy@yandex-team.ru>
Cc: 'Florian Weimer' <fweimer@redhat.com>, musl@lists.openwall.com
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User-Agent: Mutt/1.5.21 (2010-09-15)
Subject: Re: [musl] errno and swapcontext in a multithreaded setup

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.

Rich