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[2a02:3100:9c00:bb01:3966:facf:793f:82a8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49558cb8433sm6377835e9.2.2026.07.18.14.45.21 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sat, 18 Jul 2026 14:45:22 -0700 (PDT) From: Karl Mehltretter To: Russell King , Sebastian Andrzej Siewior Cc: Karl Mehltretter , Arnd Bergmann , linux-rt-devel@lists.linux.dev, linux-arm-kernel@lists.infradead.org Subject: [PATCH] ARM: select HAVE_POSIX_CPU_TIMERS_TASK_WORK Date: Sat, 18 Jul 2026 23:45:15 +0200 Message-Id: <20260718214515.11275-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Commit c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") enabled PREEMPT_RT on ARM but did not select HAVE_POSIX_CPU_TIMERS_TASK_WORK. This leaves CONFIG_POSIX_CPU_TIMERS_TASK_WORK disabled, so CPU timers expire in hard IRQ context. On PREEMPT_RT this makes run_posix_cpu_timers() take the sleeping sighand->siglock: BUG: sleeping function called from invalid context at spinlock_rt.c:48 rt_spin_lock from lock_task_sighand lock_task_sighand from run_posix_cpu_timers run_posix_cpu_timers from update_process_times ARM handles TIF_NOTIFY_RESUME on all return-to-user paths, including v7-M. ARM32 KVM host support was removed by commit 541ad0150ca4 ("arm: Remove 32bit KVM host support"), so the select need not be conditional on KVM. Select it to defer POSIX CPU timer expiry to task context. Reproduced with setrlimit(RLIMIT_CPU, ...) and a busy loop. The same path is used by setitimer(ITIMER_PROF or ITIMER_VIRTUAL) and POSIX CPU timers created with timer_create(). Fixes: c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") Assisted-by: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- Found with a self-hosted syzkaller instance patched to fuzz kernels on QEMU's versatilepb machine (ARM926EJ-S, ARMv5TE). Runtime-tested before and after the change on that setup, and compile-tested mps2_defconfig with PREEMPT_RT (NOMMU, CPU_V7M). arch/arm/Kconfig | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 9187240a02db..f073b5102d4e 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -133,6 +133,7 @@ config ARM select HAVE_PERF_EVENTS select HAVE_PERF_REGS select HAVE_PERF_USER_STACK_DUMP + select HAVE_POSIX_CPU_TIMERS_TASK_WORK select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE select HAVE_REGS_AND_STACK_ACCESS_API select HAVE_RSEQ -- 2.53.0