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McKenney" Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , David Woodhouse , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Davidlohr Bueso , Kunwu Chan , Puranjay Mohan , rcu@vger.kernel.org Subject: [PATCH 1/4] srcu: Make atomic SRCU readers NMI-safe Date: Tue, 10 Nov 2026 14:28:41 -0500 Message-Id: <20261110192844.44713-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20261110192844.44713-1-mathieu.desnoyers@efficios.com> References: <20261110192844.44713-1-mathieu.desnoyers@efficios.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Atomic SRCU readers count themselves with __srcu_read_lock(), whose this_cpu_inc() is not NMI-safe on every architecture. With CONFIG_PROVE_RCU, which lockdep enables, __srcu_check_read_flavor() also warns when an atomic reader runs in NMI. So an NMI handler cannot use atomic SRCU, while a tracer or a BPF program may need to read a structure that atomic SRCU protects. Count atomic readers with __srcu_read_lock_nmisafe() and __srcu_read_unlock_nmisafe() instead. They use NMI-safe increments on the architectures that need them, and keep the full barriers that synchronize_srcu_atomic() pairs with. The __srcu_read_lock_fast() helpers have no such barriers: they rely on a grace period that calls synchronize_rcu(), which synchronize_srcu_atomic() cannot do while it spins. Also accept the atomic flavor in NMI in __srcu_check_read_flavor(). Annotate an atomic reader in NMI as a trylock for lockdep, as srcu_read_lock_nmisafe() does. An NMI can interrupt lockdep itself, and lockdep then reports any other acquisition of a lock that a grace period has used. On the architectures that need NMI-safe counters, every atomic reader now pays an atomic increment at lock and at unlock. Suggested-by: Paul E. McKenney Signed-off-by: Mathieu Desnoyers --- include/linux/srcu.h | 18 ++++++++++++++---- kernel/rcu/srcutree.c | 3 ++- 2 files changed, 16 insertions(+), 5 deletions(-) diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 1a8a465a5650..7a338e3a63d3 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -92,7 +92,7 @@ int init_srcu_struct_atomic(struct srcu_struct *ssp); // 0x04 // SRCU-lite is no longer with us. #define SRCU_READ_FLAVOR_FAST 0x04 // srcu_read_lock_fast(), also NMI-safe. #define SRCU_READ_FLAVOR_FAST_UPDOWN 0x08 // srcu_read_lock_fast_updown(). -#define SRCU_READ_FLAVOR_ATOMIC 0x10 // srcu_read_lock_atomic(). +#define SRCU_READ_FLAVOR_ATOMIC 0x10 // srcu_read_lock_atomic(), also NMI-safe. #define SRCU_READ_FLAVOR_ALL (SRCU_READ_FLAVOR_NORMAL | SRCU_READ_FLAVOR_NMI | \ SRCU_READ_FLAVOR_FAST | SRCU_READ_FLAVOR_FAST_UPDOWN | \ SRCU_READ_FLAVOR_ATOMIC) @@ -336,6 +336,9 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) * The lock and matching srcu_read_unlock_atomic() must be invoked on * the same CPU, from the same context; passing the return value to * another task is not permitted for this flavor. + * + * This function is NMI-safe, in a manner similar to + * srcu_read_lock_nmisafe(). */ static inline int srcu_read_lock_atomic(struct srcu_struct *ssp) __acquires_shared(ssp) @@ -355,8 +358,15 @@ static inline int srcu_read_lock_atomic(struct srcu_struct *ssp) if (in_task()) non_block_start(); srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_ATOMIC); - retval = __srcu_read_lock(ssp); - srcu_lock_acquire(&ssp->dep_map); + retval = __srcu_read_lock_nmisafe(ssp); + /* + * An NMI may interrupt lockdep itself. Annotate it as a trylock + * there, as srcu_read_lock_nmisafe() does. + */ + if (in_nmi()) + rcu_try_lock_acquire(&ssp->dep_map); + else + srcu_lock_acquire(&ssp->dep_map); return retval; } @@ -563,7 +573,7 @@ static inline void srcu_read_unlock_atomic(struct srcu_struct *ssp, int idx) WARN_ON_ONCE(idx & ~0x1); srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_ATOMIC); srcu_lock_release(&ssp->dep_map); - __srcu_read_unlock(ssp, idx); + __srcu_read_unlock_nmisafe(ssp, idx); if (in_task()) non_block_end(); preempt_enable(); diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 2b0e2b2d2c18..6aebd3e31654 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -828,7 +828,8 @@ void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) /* NMI-unsafe use in NMI is a bad sign, as is multi-bit read_flavor values. */ WARN_ON_ONCE(read_flavor != SRCU_READ_FLAVOR_NMI && - read_flavor != SRCU_READ_FLAVOR_FAST && in_nmi()); + read_flavor != SRCU_READ_FLAVOR_FAST && + read_flavor != SRCU_READ_FLAVOR_ATOMIC && in_nmi()); WARN_ON_ONCE(read_flavor & (read_flavor - 1)); sdp = raw_cpu_ptr(ssp->sda); -- 2.47.3