linux-kernel - Re: [PATCH v2 04/10] um: Turn signals

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Message-ID: <b1a57068ec7578d20dcb3b28234e4ce7ec216657.camel@sipsolutions.net>
Date: Thu, 11 Sep 2025 11:44:21 +0200
From: Johannes Berg <johannes@...solutions.net>
To: Benjamin Berg <benjamin@...solutions.net>, Tiwei Bie
 <tiwei.bie@...ux.dev>
Cc: richard@....at, anton.ivanov@...bridgegreys.com, arnd@...db.de, 
	linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	tiwei.btw@...group.com
Subject: Re: [PATCH v2 04/10] um: Turn signals_* into thread-local variables

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@...group.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.

johannes