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); }

/*