linux-kernel - Re: [PATCH v2 rcu 0/8] NMI-safe SRCU reader API

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Message-ID: <20221020224146.GA354603@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 20 Oct 2022 15:41:46 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     John Ogness <john.ogness@...utronix.de>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, tglx@...utronix.de, pmladek@...e.com
Subject: Re: [PATCH v2 rcu 0/8] NMI-safe SRCU reader API

On Thu, Oct 20, 2022 at 03:27:18PM -0700, Paul E. McKenney wrote:
> On Thu, Oct 20, 2022 at 12:05:37AM +0200, Frederic Weisbecker wrote:
> > On Wed, Oct 19, 2022 at 12:14:18PM -0700, Paul E. McKenney wrote:
> > > On Wed, Oct 19, 2022 at 01:19:53PM +0206, John Ogness wrote:
> > > > On 2022-10-18, "Paul E. McKenney" <paulmck@...nel.org> wrote:
> > > > > And the v6.1-rc1 stack is now at srcunmisafe.2022.10.18b.
> > > > 
> > > > Thanks!
> > > > 
> > > > I guess the kernel test robot will catch this, but if you checkout
> > > > commit 79c95dc428ad ("arch/x86: Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
> > > > Kconfig option") and build for x86_64, you will get:
> > > > 
> > > > x86_64-linux-gnu-ld: kernel/rcu/srcutree.o: in function `srcu_gp_start_if_needed':
> > > > srcutree.c:(.text+0x133a): undefined reference to `__srcu_read_lock_nmisafe'
> > > > x86_64-linux-gnu-ld: srcutree.c:(.text+0x1490): undefined reference to `__srcu_read_unlock_nmisafe'
> > > > x86_64-linux-gnu-ld: kernel/rcu/srcutree.o: in function `srcu_barrier':
> > > > srcutree.c:(.text+0x1b03): undefined reference to `__srcu_read_lock_nmisafe'
> > > > x86_64-linux-gnu-ld: srcutree.c:(.text+0x1b38): undefined reference to `__srcu_read_unlock_nmisafe'
> > > > 
> > > > Note that this error is fixed with a later commit:
> > > > 
> > > > commit c2d158a284ab ("srcu: Debug NMI safety even on archs that don't
> > > > require it").
> > > > 
> > > > This does not affect what I am working on, so feel free to take care of
> > > > it whenever it fits your schedule.
> > > 
> > > Good catch, thank you!
> > > 
> > > It looks like the first two hunks in include/linux/srcu.h from that
> > > later commit need to be in that earlier commit.
> > > 
> > > Frederic, does this make sense, or am I off in the weeds?
> > 
> > Actually you need to do that earlier, in
> > 6584822b1be1 ("srcu: Create an srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe()")
> > 
> > This way you don't only fix x86 bisectability but also the one of all the other safe archs.
> > 
> > And it's not just the first two hunks, you also need to include
> > the removal of the srcutiny.h/srcutree.h definitions.
> > 
> > So namely you need to apply the following to 6584822b1be1. You might
> > meet some minor retro-conflicts (the chknmisafe parameter didn't exist yet),
> > but that's pretty much it:
> 
> Thank you both!
> 
> I have an untested but allegedly fixed branch on -rcu on branch
> srcunmisafe.2022.10.20a.

Aside from having accidentally fused Frederic's last two commits together.
I will split them back out on the next rebase.

							Thanx, Paul

> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index 565f60d57484..f0814ffca34b 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -52,8 +52,6 @@ int init_srcu_struct(struct srcu_struct *ssp);
> >  #else
> >  /* Dummy definition for things like notifiers.  Actual use gets link error. */
> >  struct srcu_struct { };
> > -int __srcu_read_lock_nmisafe(struct srcu_struct *ssp, bool chknmisafe) __acquires(ssp);
> > -void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx, bool chknmisafe) __releases(ssp);
> >  #endif
> >  
> >  void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
> > @@ -66,6 +64,20 @@ unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> >  unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> >  bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
> >  
> > +#ifdef CONFIG_NEED_SRCU_NMI_SAFE
> > +int __srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp);
> > +void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) __releases(ssp);
> > +#else
> > +static inline int __srcu_read_lock_nmisafe(struct srcu_struct *ssp)
> > +{
> > +	return __srcu_read_lock(ssp);
> > +}
> > +static inline void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
> > +{
> > +	__srcu_read_unlock(ssp, idx);
> > +}
> > +#endif /* CONFIG_NEED_SRCU_NMI_SAFE */
> > +
> >  #ifdef CONFIG_SRCU
> >  void srcu_init(void);
> >  #else /* #ifdef CONFIG_SRCU */
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index f890301f123d..f3a4d65b91ef 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -89,16 +89,4 @@ static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
> >  		 data_race(READ_ONCE(ssp->srcu_idx)),
> >  		 data_race(READ_ONCE(ssp->srcu_idx_max)));
> >  }
> > -
> > -static inline int __srcu_read_lock_nmisafe(struct srcu_struct *ssp, bool chknmisafe)
> > -{
> > -	BUG();
> > -	return 0;
> > -}
> > -
> > -static inline void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx, bool chknmisafe)
> > -{
> > -	BUG();
> > -}
> > -
> >  #endif
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index 35ffdedf86cc..c689a81752c9 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -159,7 +155,4 @@ void synchronize_srcu_expedited(struct srcu_struct *ssp);
> >  void srcu_barrier(struct srcu_struct *ssp);
> >  void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf);
> >  
> > -int __srcu_read_lock_nmisafe(struct srcu_struct *ssp, bool chknmisafe) __acquires(ssp);
> > -void __srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx, bool chknmisafe) __releases(ssp);
> > -
> >  #endif
> > 
> >