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McKenney" , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt Cc: linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Waiman Long Subject: [PATCH 2/2] clocksource: Defer preempt_disable() after clocksource_verify_choose_cpus() Date: Fri, 24 Jan 2025 13:59:23 -0500 Message-ID: <20250124185923.3610964-2-longman@redhat.com> In-Reply-To: <20250124185923.3610964-1-longman@redhat.com> References: <20250124185923.3610964-1-longman@redhat.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 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 The following bug report happened in a PREEMPT_RT kernel. [ 30.957705] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48 [ 30.957711] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2012, name: kwatchdog [ 30.962673] preempt_count: 1, expected: 0 [ 30.962676] RCU nest depth: 0, expected: 0 [ 30.962680] 3 locks held by kwatchdog/2012: [ 30.962684] #0: ffffffff8af2da60 (clocksource_mutex){+.+.}-{3:3}, at: clocksource_watchdog_kthread+0x13/0x50 [ 30.967703] #1: ffffffff8aa8d4d0 (cpu_hotplug_lock){++++}-{0:0}, at: clocksource_verify_percpu.part.0+0x5c/0x330 [ 30.972774] #2: ffff9fe02f5f33e0 ((batched_entropy_u32.lock)){+.+.}-{2:2}, at: get_random_u32+0x4f/0x110 [ 30.977827] Preemption disabled at: [ 30.977830] [] clocksource_verify_percpu.part.0+0x66/0x330 [ 30.982837] CPU: 33 PID: 2012 Comm: kwatchdog Not tainted 5.14.0-503.23.1.el9_5.x86_64+rt-debug #1 [ 30.982843] Hardware name: HPE ProLiant DL385 Gen10 Plus/ProLiant DL385 Gen10 Plus, BIOS A42 04/29/2021 [ 30.982846] Call Trace: [ 30.982850] [ 30.983821] dump_stack_lvl+0x57/0x81 [ 30.983821] __might_resched.cold+0xf4/0x12f [ 30.983824] rt_spin_lock+0x4c/0x100 [ 30.988833] get_random_u32+0x4f/0x110 [ 30.988833] clocksource_verify_choose_cpus+0xab/0x1a0 [ 30.988833] clocksource_verify_percpu.part.0+0x6b/0x330 [ 30.993894] __clocksource_watchdog_kthread+0x193/0x1a0 [ 30.993898] clocksource_watchdog_kthread+0x18/0x50 [ 30.993898] kthread+0x114/0x140 [ 30.993898] ret_from_fork+0x2c/0x50 [ 31.002864] It is due to the fact that get_random_u32() is called in clocksource_verify_choose_cpus() with preemption disabled. If crng_ready() is true by the time get_random_u32() is called, The batched_entropy_32 local lock will be acquired. In PREEMPT_RT kernel, it is a rtmutex and we can't acquire it with preemption disabled. To avoid this problem, we can't call get_random_u32() with preemption disabled. However, smp_processor_id() has to be called with preemption disabled though and we have to exclude the current CPU from the cpus_chosen list to be tested. Extract current CPU removal code out from clocksource_verify_choose_cpus() and defer the preempt_disable() call to after clocksource_verify_choose_cpus() and before current CPU removal. Also use raw_smp_processor_id() in clocksource_verify_choose_cpus(). Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable") Signed-off-by: Waiman Long --- kernel/time/clocksource.c | 19 +++++++++++-------- 1 file changed, 11 insertions(+), 8 deletions(-) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 77d9566d3aa6..8ffec49971ac 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -313,7 +313,6 @@ static void clocksource_verify_choose_cpus(void) if (n < 0) { /* Check all of the CPUs. */ cpumask_copy(&cpus_chosen, cpu_online_mask); - cpumask_clear_cpu(smp_processor_id(), &cpus_chosen); return; } @@ -322,9 +321,13 @@ static void clocksource_verify_choose_cpus(void) if (n == 0 || num_online_cpus() <= 1) return; - /* Make sure to select at least one CPU other than the current CPU. */ + /* + * Make sure to select at least one CPU other than the current CPU. + * raw_smp_processor_id() is used opportunistically here as + * preemption isn't disabled. + */ cpu = cpumask_first(cpu_online_mask); - if (cpu == smp_processor_id()) + if (cpu == raw_smp_processor_id()) cpu = cpumask_next(cpu, cpu_online_mask); if (WARN_ON_ONCE(cpu >= nr_cpu_ids)) return; @@ -349,9 +352,6 @@ static void clocksource_verify_choose_cpus(void) if (!WARN_ON_ONCE(cpu >= nr_cpu_ids)) cpumask_set_cpu(cpu, &cpus_chosen); } - - /* Don't verify ourselves. */ - cpumask_clear_cpu(smp_processor_id(), &cpus_chosen); } static void clocksource_verify_one_cpu(void *csin) @@ -373,15 +373,18 @@ void clocksource_verify_percpu(struct clocksource *cs) cpumask_clear(&cpus_ahead); cpumask_clear(&cpus_behind); cpus_read_lock(); - preempt_disable(); clocksource_verify_choose_cpus(); + preempt_disable(); + + /* Don't verify ourselves. */ + testcpu = smp_processor_id(); + cpumask_clear_cpu(testcpu, &cpus_chosen); if (cpumask_empty(&cpus_chosen)) { preempt_enable(); cpus_read_unlock(); pr_warn("Not enough CPUs to check clocksource '%s'.\n", cs->name); return; } - testcpu = smp_processor_id(); pr_info("Checking clocksource %s synchronization from CPU %d to CPUs %*pbl.\n", cs->name, testcpu, cpumask_pr_args(&cpus_chosen)); for_each_cpu(cpu, &cpus_chosen) { -- 2.47.1