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[2a02:3100:b149:1401:90c6:65b4:35d:1e3d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b7da62edb4sm118646645e9.1.2026.11.07.11.49.42 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sat, 07 Nov 2026 11:49:43 -0800 (PST) From: Karl Mehltretter To: Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti Cc: Karl Mehltretter , Charlie Jenkins , Thomas Gleixner , Steven Rostedt , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: [PATCH] riscv: entry: Mark ret_from_fork_kernel() and ret_from_fork_user() noinstr Date: Sat, 7 Nov 2026 20:49:31 +0100 Message-Id: <20261107194931.69176-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) 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 ret_from_fork_kernel() and ret_from_fork_user() are traceable and return after syscall_exit_to_user_mode() enters the RCU extended quiescent state via user_enter_irqoff() on NO_HZ_FULL CPUs. When function_graph tracing is enabled with FTRACE_VALIDATE_RCU_IS_WATCHING, the return hook runs after RCU stops watching and triggers the ftrace_return_to_handler() warning. Mark ret_from_fork_kernel() and ret_from_fork_user() noinstr and bracket the kernel thread callback with instrumentation_begin() and instrumentation_end(). Fixes: f955aa8723a6 ("riscv: entry: Convert ret_from_fork() to C") Signed-off-by: Karl Mehltretter --- Assisted by an LLM. Tested on v7.4-rc1 (d48723711bae) under QEMU riscv64 (-smp 4) with NO_HZ_FULL, FTRACE_VALIDATE_RCU_IS_WATCHING, PROVE_LOCKING, nohz_full=1-3 rcu_nocbs=1-3 on the kernel command line, and function_graph tracing during parallel fork bursts across CPUs 0-3. Without this patch, two consecutive runs trigger: RCU not on for: ret_from_fork_user+0x4/0x368 WARNING: include/linux/trace_recursion.h:132 at ftrace_return_to_handler+0x2e0/0x2f0, CPU#2 followed by an observed kernel paging request Oops in ftrace_graph_ret_addr() while unwinding the warning backtrace. With this patch, two consecutive runs complete without the ftrace_return_to_handler() warning or unwind fault. arch/riscv/kernel/process.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index 34c1d498d1cad..10c1bee77145f 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -225,14 +225,17 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) return 0; } -asmlinkage void ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), struct pt_regs *regs) +asmlinkage void noinstr ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), + struct pt_regs *regs) { + instrumentation_begin(); fn(fn_arg); + instrumentation_end(); syscall_exit_to_user_mode(regs); } -asmlinkage void ret_from_fork_user(struct pt_regs *regs) +asmlinkage void noinstr ret_from_fork_user(struct pt_regs *regs) { syscall_exit_to_user_mode(regs); } base-commit: d48723711bae9b8cdc56719c448ba42fef28cbee -- 2.39.5 (Apple Git-154)