RE: [musl] errno and swapcontext in a multithreaded setup

["Andrey Bugaevskiy" <bugaevskiy@yandex-team.ru>]

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