Re: [PATCH v2 04/10] um: Turn signals_* into thread-local variables

[Benjamin Berg <benjamin@sipsolutions.net>]

On Thu, 2025-09-11 at 11:44 +0200, Johannes Berg wrote:
> On Thu, 2025-09-11 at 09:37 +0200, Benjamin Berg wrote:
> > Hi,
> > 
> > On Thu, 2025-09-11 at 12:34 +0800, Tiwei Bie wrote:
> > > On Wed, 10 Sep 2025 14:15:28 +0200, Johannes Berg wrote:
> > > > On Sun, 2025-08-10 at 13:51 +0800, Tiwei Bie wrote:
> > > > > From: Tiwei Bie <tiwei.btw@antgroup.com>
> > > > > 
> > > > > Turn signals_enabled, signals_pending and signals_active into
> > > > > thread-local variables. This enables us to control and track
> > > > > signals independently on each CPU thread. This is a preparation
> > > > > for adding SMP support.
> > > > 
> > > > [...]
> > > > 
> > > > > +static __thread int signals_enabled;
> > > > 
> > > > How much glibc infrastructure does __thread rely on? More
> > > > specifically:
> > > > Some time ago we had a discussion about building UML as a nolibc
> > > > binary,
> > > > what would that mean for the __thread usage here?
> > > 
> > > We would need to parse TLS data (PT_TLS) from the ELF file ourselves
> > > and properly set up TLS when creating threads using clone().
> > 
> > I guess right now we cannot use PER_CPU variables in these files.
> 
> Maybe? The only thing would be to know which "CPU" we're executing on?
> getpid() is async signal safe (i.e. you can call it), but there could be
> better ways of doing this such as setting different signal handler
> functions in different CPUs.

There are a number of ways to solve that problem.

One way would be to use a signal stack and calculating it from the
stack pointer. Though I am still considering that we should use the
tasks stack as the signal stack, in which case we may need to track the
CPU that a task is executing on.

On 64bit, one could also use the FS/GS registers for per-CPU data. I
believe the libc uses FS only on 64bit, so we could probably already
use the GS register to for per-CPU data.

So, I am not really worried about this, we probably need a nice
solution for per-CPU data anyway.

Benjamin