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

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Message-ID: <2067a019-7a85-4b9f-b162-b2cbcdd006ec@paulmck-laptop>
Date: Tue, 22 Oct 2024 07:27:24 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, andrii@...nel.org
Subject: Re: [PATCH rcu] srcu: Guarantee non-negative return value from
 srcu_read_lock()

On Tue, Oct 22, 2024 at 09:07:17AM +0200, Peter Zijlstra wrote:
> On Mon, Oct 21, 2024 at 08:30:43PM -0700, Paul E. McKenney wrote:
> > Thoughts?
> 
> Thanks Paul!
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thank you, Peter!  I will apply this on my next rebase.

							Thanx, Paul

> > ------------------------------------------------------------------------
> > 
> > commit 4433b7d3785d8d2a700f5ed5ca234c64bc63180e
> > Author: Paul E. McKenney <paulmck@...nel.org>
> > Date:   Mon Oct 21 15:09:39 2024 -0700
> > 
> >     srcu: Guarantee non-negative return value from srcu_read_lock()
> >     
> >     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, and does the same for the underlying
> >     __srcu_read_lock().
> >     
> >     [ paulmck: Apply Andrii Nakryiko feedback. ]
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >     Cc: Peter Zijlstra <peterz@...radead.org>
> >     Acked-by: 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.
> >   */
> >  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..ae17c214e0de5 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)
> >  {