linux-kernel - Re: [PATCH] futex: Fix a faulty comment.

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Message-ID: <CAF=mnpi65WtnK_mF1f8GfMEczS1ruj8q4YbA18R5KvWcpkxLxw@mail.gmail.com>
Date:   Tue, 7 Dec 2021 02:44:36 +0900
From:   Jangwoong Kim <6812skiii@...il.com>
To:     André Almeida <andrealmeid@...labora.com>
Cc:     peterz@...radead.org, mingo@...hat.com, tglx@...utronix.de,
        dvhart@...radead.org, dave@...olabs.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] futex: Fix a faulty comment.

Hi André.

That patch was definitely wrong, I apologize.

However, since futex_wait_multiple_setup() sets the last index of
futex that was woken up,
shouldn't the comment be modified as below?

If so, I will resend a patch.

                /*
-                * Even if something went wrong, if we find out that a futex
-                * was woken, we don't return error and return this index to
-                * userspace
+                * Even if something went wrong, if we find out that any futex
+                * was woken, we don't return error and return the last index
+                * awoken to userspace
                 */
                *woken = unqueue_multiple(vs, i);
                if (*woken >= 0)

I sent the patch because I thought this was important enough to be corrected.

Let me know If this is not crucial enough to be patched, so I won't
keep sending comment-fixing patches.

Thank you.
Jangwoong Kim.

2021년 12월 6일 (월) 오후 9:12, André Almeida <andrealmeid@...labora.com>님이 작성:
>
> Hi Jangwoong,
>
> Thanks for your patch! However...
>
> Às 15:14 de 04/12/21, 6812skiii@...il.com escreveu:
> > From: Jangwoong Kim <6812skiii@...il.com>
> >
> > We return 1, not the index of futex woken up.
> >
> > Signed-off-by: Jangwoong Kim <6812skiii@...il.com>
> > ---
> >  kernel/futex/waitwake.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
> > index 4ce0923f1ce3..d148e5d4956b 100644
> > --- a/kernel/futex/waitwake.c
> > +++ b/kernel/futex/waitwake.c
> > @@ -455,8 +455,8 @@ static int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *wo
> >
> >               /*
> >                * Even if something went wrong, if we find out that a futex
> > -              * was woken, we don't return error and return this index to
> > -              * userspace
> > +              * was woken, we don't return error and make userspace aware
> > +              * of this by returning 1.
>
> We return to userspace the value at *woken, so your fix is wrong. Have a
> look at futex_wait_multiple():
>
>                 ret = futex_wait_multiple_setup(vs, count, &hint);
>                 if (ret) {
>                         if (ret > 0) {
>                                 /* A futex was woken during setup */
>                                 ret = hint;
>                         }
>                         return ret;
>                 }
>
> When we return 1 at futex_wait_multiple_setup(), we end up returning the
> hint/woken value to userspace.
>
> Let me know if you have questions.
>
>         André