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

Joel Fernandes
2022-10-14T22:47:40Z

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().

Just a comment about high-level use of the new NMI-safe SRCU api: say if for
certain arch, NMI cannot be interrupted (by breakpoint or something), then
using NMI-safe API will force such arch to potentially use more expensive RMW
atomic than the previously cheap local non-atomic operations that the arch
was able to get away with.

Thoughts? Otherwise, LGTM.

thanks,

- Joel

> 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(-)