linux-kernel - Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read

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Message-ID: <CAEf4BzbX5UtS=+Np5t68n8oRE2O2qGg5iDcikNFpvCTPBZU8kg@mail.gmail.com>
Date: Mon, 21 Oct 2024 19:01:02 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com, 
	rostedt@...dmis.org, peterz@...radead.org, andrii@...nel.org
Subject: Re: [PATCH rcu] srcu: Guarantee non-negative return value from srcu_read_lock()

On Mon, Oct 21, 2024 at 5:21 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Mon, Oct 21, 2024 at 04:50:44PM -0700, Andrii Nakryiko wrote:
> > On Mon, Oct 21, 2024 at 3:13 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > >
> > > For almost 20 years, the int return value from srcu_read_lock() has
> > > been always either zero or one.  This commit therefore documents the
> > > fact that it will be non-negative.
> > >
> > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > Cc: Andrii Nakryiko <andrii@...nel.org
> > >
> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > index bab1dae3f69e6..512a8c54ba5ba 100644
> > > --- a/include/linux/srcu.h
> > > +++ b/include/linux/srcu.h
> > > @@ -238,13 +238,14 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
> > >   * a mutex that is held elsewhere while calling synchronize_srcu() or
> > >   * synchronize_srcu_expedited().
> > >   *
> > > - * The return value from srcu_read_lock() must be passed unaltered
> > > - * to the matching srcu_read_unlock().  Note that srcu_read_lock() and
> > > - * the matching srcu_read_unlock() must occur in the same context, for
> > > - * example, it is illegal to invoke srcu_read_unlock() in an irq handler
> > > - * if the matching srcu_read_lock() was invoked in process context.  Or,
> > > - * for that matter to invoke srcu_read_unlock() from one task and the
> > > - * matching srcu_read_lock() from another.
> > > + * The return value from srcu_read_lock() is guaranteed to be
> > > + * non-negative.  This value must be passed unaltered to the matching
> > > + * srcu_read_unlock().  Note that srcu_read_lock() and the matching
> > > + * srcu_read_unlock() must occur in the same context, for example, it is
> > > + * illegal to invoke srcu_read_unlock() in an irq handler if the matching
> > > + * srcu_read_lock() was invoked in process context.  Or, for that matter to
> > > + * invoke srcu_read_unlock() from one task and the matching srcu_read_lock()
> > > + * from another.
> >
> > For uprobe work I'm using __srcu_read_lock() and __srcu_read_unlock().
> > Presumably the same non-negative index will be returned/consumed there
> > as well, right? Can we add a blurb to that effect for them as well?
>
> Does the change shown below cover it?

Yep, looks good, thank you! You might want to fix
s/srcu_read_unlock/__srcu_read_unlock/, while at it, but that's
orthogonal.

>
> > Otherwise LGTM, thanks!
> >
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
>
> Thank you -- I will apply on my next rebase.
>
>                                                 Thanx, Paul
>
> > >   */
> > >  static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
> > >  {
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 07147efcb64d3..3d587bf2b2c12 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -738,7 +738,8 @@ EXPORT_SYMBOL_GPL(srcu_check_read_flavor);
>  /*
>   * Counts the new reader in the appropriate per-CPU element of the
>   * srcu_struct.
> - * Returns an index that must be passed to the matching srcu_read_unlock().
> + * Returns a guaranteed non-negative index that must be passed to the
> + * matching srcu_read_unlock().
>   */
>  int __srcu_read_lock(struct srcu_struct *ssp)
>  {