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

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Date:   Mon, 3 Oct 2022 09:38:50 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org, tglx@...utronix.de,
        john.ogness@...utronix.de, pmladek@...e.com
Subject: Re: [PATCH v2 rcu 0/8] NMI-safe SRCU reader API

On Mon, Oct 03, 2022 at 04:11:41PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 29, 2022 at 11:07:14AM -0700, Paul E. McKenney wrote:
> > Hello!
> > 
> > This RFC series provides the second version of an NMI-safe SRCU reader API
> > in the guise of srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe().
> > A given srcu_struct structure must use either the traditional
> > srcu_read_lock() and srcu_read_unlock() API or the new _nmisafe() API:
> > Mixing and matching is not permitted.  So much so that kernels built
> > with CONFIG_PROVE_RCU=y will complain if you try it.
> > 
> > The reason for this restriction is that I have yet to find a use case
> > that is not a accident waiting to happen.  And if free intermixing
> > were permitted, it is pretty much a given that someone somewhere will
> > get confused and use srcu_read_lock_nmisafe() within NMI handlers and
> > srcu_read_lock() elsewhere, which will not (repeat, NOT) provide NMI
> > safety.
> > 
> > I do not expect to push this into the v6.1 merge window.  However, if
> > the printk() series that needs it goes in, then I will push it as a fix
> > for the resulting regression.
> > 
> > The series is as follows:
> > 
> > 1.	Convert ->srcu_lock_count and ->srcu_unlock_count to atomic.
> > 
> > 2.	Create an srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe().
> > 
> > 3.	Check for consistent per-CPU per-srcu_struct NMI safety.
> > 
> > 4.	Check for consistent global per-srcu_struct NMI safety.
> > 
> > 5.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.
> > 
> > 6.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.
> > 
> > 7.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.
> > 
> > 8.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.
> > 
> > Changes since v1 RFC:
> > 
> > 1.	Added enabling patches for arm64, loongarch, s390, and x86.
> > 	These have what appear to me to be NMI-safe this_cpu_inc()
> > 	implementations.
> > 
> > 2.	Fix a build error on !SMP kernels built without SRCU.
> > 
> > 3.	Fix a build error on !SMP kernels.
> > 
> > 						Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> >  b/arch/arm64/Kconfig       |    1 
> >  b/arch/loongarch/Kconfig   |    1 
> >  b/arch/s390/Kconfig        |    1 
> >  b/arch/x86/Kconfig         |    1 
> >  b/include/linux/srcu.h     |   39 +++++++++++++++++++++
> >  b/include/linux/srcutiny.h |   11 ++++++
> >  b/include/linux/srcutree.h |    4 +-
> >  b/kernel/rcu/Kconfig       |    3 +
> >  b/kernel/rcu/rcutorture.c  |   11 ++++--
> >  b/kernel/rcu/srcutree.c    |   24 ++++++-------
> >  include/linux/srcu.h       |    4 +-
> >  include/linux/srcutiny.h   |    4 +-
> >  include/linux/srcutree.h   |   12 +++++-
> >  kernel/rcu/srcutree.c      |   82 +++++++++++++++++++++++++++++++++++++++------
> >  14 files changed, 166 insertions(+), 32 deletions(-)
> 
> Except for patches 6/7/8, for which I may miss subtle things:
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Applied, thank you!

The new -rcu branch is srcunmisafe.2022.10.03a.

							Thanx, Paul