Re: [PATCH] vdso/gettimeofday: Reload sequence counter after switch to time page in do_aux()

[Thomas Weißschuh]

Hi Ricardo,

On Mon, Apr 27, 2026 at 08:09:49PM +0800, Ricardo Ribalda wrote:
> Thanks for the patch. Any idea when do you plan to land this?

Thomas Gleixner will pick it up as soon as he gets to it.
That is if he has no objections to the patch itself.
I trust that it will be fairly soon.

Can you confirm that the patch fixes the warning you reported originally?


Thomas

> On Wed, 22 Apr 2026 at 11:49, Christophe Leroy (CS GROUP)
> <chleroy@xxxxxxxxxx> wrote:
> >
> >
> >
> > Le 22/04/2026 à 11:42, Thomas Weißschuh a écrit :
> > > After switching to the real data pages, the sequence counter needs to be
> > > reloaded from there. The code using vdso_read_begin_timens() assumed
> > > this worked by 'continue' jumping to the *beginning* of the do-while
> > > retry loop. However the 'continue' jumps to the *end* of said loop,
> > > evaluating the exit condition. If the data page has a sequence counter
> > > of '1' it will match the one from the time namespace page and prematurely
> > > exit the retry loop. This would result in garbage returned to the caller.
> > >
> > > Reload the sequence counter after switching the pages by using an inner
> > > while loop again, which will loop at most once.
> > >
> > > The loop generates slightly better code than an explicit reload through
> > > 'seq = vdso_read_begin()'.
> > >
> > > Reported-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > > Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2FCANiDSCsOy0P1if-gJZqOM5pTJ0RDcwVfru1B7KFbTOEMqjPKJw%40mail.gmail.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C8090d715bf644286a5e908dea0538801%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639124477796711565%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=yQti2tvfaExxkn%2BoD7wzPhAkFTYQ7Sr%2FV3e3fRdJYVY%3D&reserved=0
> > > Fixes: ed78b7b2c5ae ("vdso/gettimeofday: Add a helper to read the sequence lock of a time namespace aware clock")
> > > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
> >
> > Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@xxxxxxxxxx>
> >
> > > ---
> > >   lib/vdso/gettimeofday.c | 5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
> > > index a5798bd26d20..da224011fafd 100644
> > > --- a/lib/vdso/gettimeofday.c
> > > +++ b/lib/vdso/gettimeofday.c
> > > @@ -248,11 +248,10 @@ bool do_aux(const struct vdso_time_data *vd, clockid_t clock, struct __kernel_ti
> > >       vc = &vd->aux_clock_data[idx];
> > >
> > >       do {
> > > -             if (vdso_read_begin_timens(vc, &seq)) {
> > > +             while (vdso_read_begin_timens(vc, &seq)) {
> > > +                     /* Re-read from the real time data page, reload seq by looping */
> > >                       vd = __arch_get_vdso_u_timens_data(vd);
> > >                       vc = &vd->aux_clock_data[idx];
> > > -                     /* Re-read from the real time data page */
> > > -                     continue;
> > >               }
> > >
> > >               /* Auxclock disabled? */
> > >
> > > ---
> > > base-commit: aa70a22882fe51fadd2392b480864d15e1b9ab3d
> > > change-id: 20260422-vdso-aux-timens-loop-acbaa5a2434e
> > >
> > > Best regards,
> > > --
> > > Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
> > >
> >
> 
> 
> --
> Ricardo Ribalda