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

Paul E. McKenney
2022-10-03T16:38:58Z

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@kernel.org>

Applied, thank you!

The new -rcu branch is srcunmisafe.2022.10.03a.

Thanx, Paul