Re: [RFC] linux-user: Remove stale "not threadsafe" comments

[Warner Losh <imp@bsdimp.com>]

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On Sat, Jan 15, 2022 at 2:49 AM Alex Bennée <alex.bennee@linaro.org> wrote:

>
> Peter Maydell <peter.maydell@linaro.org> writes:
>
> > In linux-user/signal.c we have two FIXME comments claiming that
> > parts of the signal-handling code are not threadsafe. These are
> > very old, as they were first introduced in commit 624f7979058
> > in 2008. Since then we've radically overhauled the signal-handling
> > logic, while carefully preserving these FIXME comments.
> >
> > It's unclear exactly what thread-safety issue the original
> > author was trying to point out -- the relevant data structures
> > are in the TaskStruct, which makes them per-thread and only
> > operated on by that thread. The old code at the time of that
> > commit did have various races involving signal handlers being
> > invoked at awkward times; possibly this was what was meant.
> >
> > Delete these FIXME comments:
> >  * they were written at a time when the way we handled
> >    signals was completely different
> >  * the code today appears to us to not have thread-safety issues
> >  * nobody knows what the problem the comments were trying to
> >    point out was
> > so they are serving no useful purpose for us today.
> >
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> > Marked "RFC" because I'm a bit uneasy with deleting FIXMEs
> > simply because I can't personally figure out why they're
> > there. This patch is more to start a discussion to see
> > if anybody does understand the issue -- in which case we
> > can instead augment the comments to describe it.
> > ---
> >  linux-user/signal.c | 2 --
> >  1 file changed, 2 deletions(-)
>




> > diff --git a/linux-user/signal.c b/linux-user/signal.c
> > index 32854bb3752..e7410776e21 100644
> > --- a/linux-user/signal.c
> > +++ b/linux-user/signal.c
> > @@ -1001,7 +1001,6 @@ int do_sigaction(int sig, const struct
> target_sigaction *act,
> >          oact->sa_mask = k->sa_mask;
> >      }
> >      if (act) {
> > -        /* FIXME: This is not threadsafe.  */
> >          __get_user(k->_sa_handler, &act->_sa_handler);
> >          __get_user(k->sa_flags, &act->sa_flags);
> >  #ifdef TARGET_ARCH_HAS_SA_RESTORER
> > @@ -1151,7 +1150,6 @@ void process_pending_signals(CPUArchState *cpu_env)
> >      sigset_t *blocked_set;
> >
> >      while (qatomic_read(&ts->signal_pending)) {
> > -        /* FIXME: This is not threadsafe.  */
> >          sigfillset(&set);
> >          sigprocmask(SIG_SETMASK, &set, 0);
>
> Looking at the history those FIXMEs could have been for code that they
> where attached to. Could the thread safety be about reading the
> sigaction stuff? I would have though sigaction updates where atomic by
> virtue of the syscall to set them...
>
> Anyway looks old to me:
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>

Reviewed-by: Warner Losh <imp@bsdimp.com>

I looked in bsd-user, to where this was also copied, and couldn't figure out
what it was talking about...  Though that's a weak review, imho..


> --
> Alex Bennée
>
>

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