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Date:   Thu, 26 Oct 2023 09:39:20 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>
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Subject: [tip: x86/urgent] x86/tsc: Defer marking TSC unstable to a worker
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Thomas Gleixner <tglx@linutronix.de>, stable@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org
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The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     4f0d5e708ef88b58c954581ca54161064beaaaf5
Gitweb:        https://git.kernel.org/tip/4f0d5e708ef88b58c954581ca54161064beaaaf5
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Wed, 25 Oct 2023 23:31:35 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 26 Oct 2023 11:31:45 +02:00

x86/tsc: Defer marking TSC unstable to a worker

Tetsuo reported the following lockdep splat when the TSC synchronization
fails during CPU hotplug:

   tsc: Marking TSC unstable due to check_tsc_sync_source failed
  
   WARNING: inconsistent lock state
   inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
   ffffffff8cfa1c78 (watchdog_lock){?.-.}-{2:2}, at: clocksource_watchdog+0x23/0x5a0
   {IN-HARDIRQ-W} state was registered at:
     _raw_spin_lock_irqsave+0x3f/0x60
     clocksource_mark_unstable+0x1b/0x90
     mark_tsc_unstable+0x41/0x50
     check_tsc_sync_source+0x14f/0x180
     sysvec_call_function_single+0x69/0x90

   Possible unsafe locking scenario:
     lock(watchdog_lock);
     <Interrupt>
       lock(watchdog_lock);

   stack backtrace:
    _raw_spin_lock+0x30/0x40
    clocksource_watchdog+0x23/0x5a0
    run_timer_softirq+0x2a/0x50
    sysvec_apic_timer_interrupt+0x6e/0x90

The reason is the recent conversion of the TSC synchronization function
during CPU hotplug on the control CPU to a SMP function call. In case
that the synchronization with the upcoming CPU fails, the TSC has to be
marked unstable via clocksource_mark_unstable().

clocksource_mark_unstable() acquires 'watchdog_lock', but that lock is
taken with interrupts enabled in the watchdog timer callback to minimize
interrupt disabled time. That's obviously a possible deadlock scenario,

Before that change the synchronization function was invoked in thread
context so this could not happen.

As it is not crucical whether the unstable marking happens slightly
delayed, defer the call to a worker thread which avoids the lock context
problem.

Fixes: 9d349d47f0e3 ("x86/smpboot: Make TSC synchronization function call based")
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/87zg064ceg.ffs@tglx
---
 arch/x86/kernel/tsc_sync.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index bbc440c..1123ef3 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -15,6 +15,7 @@
  * ( The serial nature of the boot logic and the CPU hotplug lock
  *   protects against more than 2 CPUs entering this code. )
  */
+#include <linux/workqueue.h>
 #include <linux/topology.h>
 #include <linux/spinlock.h>
 #include <linux/kernel.h>
@@ -342,6 +343,13 @@ static inline unsigned int loop_timeout(int cpu)
 	return (cpumask_weight(topology_core_cpumask(cpu)) > 1) ? 2 : 20;
 }
 
+static void tsc_sync_mark_tsc_unstable(struct work_struct *work)
+{
+	mark_tsc_unstable("check_tsc_sync_source failed");
+}
+
+static DECLARE_WORK(tsc_sync_work, tsc_sync_mark_tsc_unstable);
+
 /*
  * The freshly booted CPU initiates this via an async SMP function call.
  */
@@ -395,7 +403,7 @@ retry:
 			"turning off TSC clock.\n", max_warp);
 		if (random_warps)
 			pr_warn("TSC warped randomly between CPUs\n");
-		mark_tsc_unstable("check_tsc_sync_source failed");
+		schedule_work(&tsc_sync_work);
 	}
 
 	/*