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[2a02:3100:b305:2001:39fa:3d24:821a:4eae]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a0394e6c5asm281196795e9.2.2026.10.05.11.58.22 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Mon, 05 Oct 2026 11:58:23 -0700 (PDT) From: Karl Mehltretter To: Andrew Morton Cc: Karl Mehltretter , Andrey Konovalov , Alexander Potapenko , Dmitry Vyukov , Marco Elver , Bradley Morgan , Anna-Maria Behnsen , Frederic Weisbecker , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Sebastian Andrzej Siewior , Clark Williams , linux-rt-devel@lists.linux.dev, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org Subject: [PATCH v4 3/6] hrtimer: Pause KCOV during deferred rearm Date: Mon, 5 Oct 2026 20:58:00 +0200 Message-Id: <20261005185803.39170-4-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20261005185803.39170-1-kmehltretter@gmail.com> References: <20261005185803.39170-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Deferred hrtimer rearm can run after HARDIRQ_OFFSET is dropped. in_task() is then true, so KCOV attributes the instrumented timer-reprogramming subtree to current. With CONFIG_KCOV_SELFTEST added to x86_64 defconfig, the interrupt selftest fails under QEMU, detecting spurious coverage in __hrtimer_rearm_deferred(). The same happens on s390, RISC-V and LoongArch, which also enable HRTIMER_REARM_DEFERRED. Excluding the involved files instead would cost their coverage on real task-context paths, e.g. the hrtimer and timekeeping syscalls. Take the kcov_pause guard in an __always_inline wrapper around __hrtimer_rearm_deferred(). Use it at all call sites, including hrtick_schedule_exit(). Callers that may run with KCOV enabled for current are built without KCOV instrumentation or marked noinstr. HAVE_NOINSTR_HACK covers pre-GCC-12 x86. The other affected architectures restrict KCOV to GCC 12 or Clang through ARCH_WANTS_NO_INSTR. This avoids relying on __no_sanitize_coverage, which is empty before GCC 12. Tested with GCC 8.1 and 15 on x86_64. A later patch also pauses __schedule(); keeping hrtick_schedule_exit() guarded here makes the deferred-rearm fix independent of that scheduler change. Fixes: 15dd3a948855 ("hrtimer: Push reprogramming timers into the interrupt return path") Assisted-by: LLM Signed-off-by: Karl Mehltretter --- This patch depends on patch 2, "kcov: Add a kcov_pause guard". include/linux/hrtimer_rearm.h | 18 ++++++++++++++++-- kernel/sched/core.c | 2 +- 2 files changed, 17 insertions(+), 3 deletions(-) diff --git a/include/linux/hrtimer_rearm.h b/include/linux/hrtimer_rearm.h index 17a81826bd9a..cd935bab3da3 100644 --- a/include/linux/hrtimer_rearm.h +++ b/include/linux/hrtimer_rearm.h @@ -6,12 +6,25 @@ #ifdef CONFIG_HRTIMER_REARM_DEFERRED #include +#include #include #include #include void __hrtimer_rearm_deferred(void); +/* + * KCOV: Pause outside __hrtimer_rearm_deferred() to suppress entry coverage. + * Callers with KCOV enabled for current must be built without KCOV + * instrumentation. + */ +static __always_inline void hrtimer_rearm_deferred_kcov_paused(void) +{ + guard(kcov_pause)(); + + __hrtimer_rearm_deferred(); +} + /* * This is purely CPU local, so check the TIF bit first to avoid the overhead of * the atomic test_and_clear_bit() operation for the common case where the bit @@ -43,7 +56,7 @@ hrtimer_rearm_deferred_user_irq(unsigned long *tif_work, const unsigned long tif */ if (unlikely((*tif_work & TIF_REARM_MASK) == _TIF_HRTIMER_REARM)) { clear_thread_flag(TIF_HRTIMER_REARM); - __hrtimer_rearm_deferred(); + hrtimer_rearm_deferred_kcov_paused(); /* Don't go into the loop if HRTIMER_REARM was the only flag */ *tif_work &= ~TIF_HRTIMER_REARM; return !*tif_work; @@ -55,7 +68,7 @@ hrtimer_rearm_deferred_user_irq(unsigned long *tif_work, const unsigned long tif static __always_inline void hrtimer_rearm_deferred_tif(unsigned long tif_work) { if (hrtimer_test_and_clear_rearm_deferred_tif(tif_work)) - __hrtimer_rearm_deferred(); + hrtimer_rearm_deferred_kcov_paused(); } /* @@ -78,6 +91,7 @@ static __always_inline bool hrtimer_test_and_clear_rearm_deferred(void) #else /* CONFIG_HRTIMER_REARM_DEFERRED */ static __always_inline void __hrtimer_rearm_deferred(void) { } +static __always_inline void hrtimer_rearm_deferred_kcov_paused(void) { } static __always_inline void hrtimer_rearm_deferred(void) { } static __always_inline void hrtimer_rearm_deferred_tif(unsigned long tif_work) { } static __always_inline bool diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 1fe40de6ebe3..87cff2884980 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1020,7 +1020,7 @@ static inline void hrtick_schedule_exit(struct rq *rq) } if (rq->hrtick_sched & HRTICK_SCHED_REARM_HRTIMER) - __hrtimer_rearm_deferred(); + hrtimer_rearm_deferred_kcov_paused(); rq->hrtick_sched = HRTICK_SCHED_NONE; } -- 2.53.0