linux-kernel - Re: [PATCH v3 2/2] clocksource: Use get_random_bytes() in clocksource_verify_choose

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Message-ID: <20250130073704.2h2vRBTJ@linutronix.de>
Date: Thu, 30 Jan 2025 08:37:04 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Waiman Long <longman@...hat.com>
Cc: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...nel.org>, Feng Tang <feng.tang@...el.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH v3 2/2] clocksource: Use get_random_bytes() in
 clocksource_verify_choose_cpus()

On 2025-01-29 17:40:01 [-0500], Waiman Long wrote:

Instead of the backtrace

> 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] [<ffffffff88c1fe56>] 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]  <TASK>
> [   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]  </TASK>

which is huge and doesn't provide much unique information could please

> It is due to the fact that get_random_u32() is called in
> clocksource_verify_choose_cpus() with preemption disabled.  The
> batched_entropy_32 local lock and/or the base_crng.lock spinlock will
> be acquired. In PREEMPT_RT kernel, they are rtmutexes and the above
> warning will be printed if the fast path fails because of contention.

extend this? 
clocksource_verify_choose_cpus() is invoked with preemption disabled, it
invokes get_random_u32() to obtain random numbers. To do so it acquires
the lock batched_entropy_32 which is a local_lock_t. This lock becomes a
sleeping lock on PREEMPT_RT and must no be acquired in atomic context.

> Fix this problem by moving the clocksource_verify_choose_cpus() call
> before preempt_disable() while moving the part that needs preemption to
> be disabled out into a new clocksource_verify_fixup_cpus() helper that
> is called after preempt_disable(). In that way, the get_random_u32()
> function will now be called with preemption enabled.

Could you replace the patch below with
  https://lore.kernel.org/all/20250129202909.GQNNqNoH@linutronix.de/

Or is there anything that makes it not work? It looks way simpler. Just
to disable preemption during the measurement and keep the task on the
same CPU for the whole time.

> Fixes: 7560c02bdffb ("clocksource: Check per-CPU clock synchronization when marked unstable")
> Signed-off-by: Waiman Long <longman@...hat.com>

Sebastian