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(-)
