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[34.150.221.228]) by smtp.gmail.com with ESMTPSA id d9-20020ac85349000000b00343057845f7sm2800923qto.20.2022.10.14.15.47.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 14 Oct 2022 15:47:34 -0700 (PDT) Date: Fri, 14 Oct 2022 22:47:34 +0000 From: Joel Fernandes To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, tglx@linutronix.de, john.ogness@linutronix.de, pmladek@suse.com Subject: Re: [PATCH v2 rcu 0/8] NMI-safe SRCU reader API Message-ID: References: <20220921144620.GA1200846@paulmck-ThinkPad-P17-Gen-1> <20220929180714.GA2874192@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220929180714.GA2874192@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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(-)