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

Paul E. McKenney
2022-09-29T18:07:23Z

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(-)