The clocksource watchdog marks clocksources unstable from within a timer
handler.  On x86, this marking involves an on_each_cpu_cond_mask(),
which in turn invokes smp_call_function_many_cond(), which may not be
invoked from a timer handler.  Doing so results in:

WARNING: CPU: 3 PID: 0 at kernel/smp.c:815 smp_call_function_many_cond+0x46b/0x4c0

Fix this by deferring the marking to the clocksource watchdog kthread.
Note that marking unstable is already deferred, so deferring it a bit
more should be just fine.

Reported-by: Christian Heusel <christian@heusel.eu>
Closes: https://lore.kernel.org/all/46a200b4-1293-413c-8b73-37f7b50abd1a@heusel.eu/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>

---
 include/linux/clocksource.h |    1 +
 kernel/time/clocksource.c   |   16 ++++++++++++----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 65b7c41471c39..a5317a4c07990 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -149,6 +149,7 @@ struct clocksource {
 #define CLOCK_SOURCE_SUSPEND_NONSTOP		0x80
 #define CLOCK_SOURCE_RESELECT			0x100
 #define CLOCK_SOURCE_VERIFY_PERCPU		0x200
+#define CLOCK_SOURCE_DEFERRED_UNSTABLE		0x400
 /* simplify initialization of mask field */
 #define CLOCKSOURCE_MASK(bits) GENMASK_ULL((bits) - 1, 0)
 
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 7304d7cf47f2d..60ace6f64c761 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -201,7 +201,8 @@ static void clocksource_change_rating(struct clocksource *cs, int rating)
 
 static void __clocksource_unstable(struct clocksource *cs)
 {
-	cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLOCK_SOURCE_WATCHDOG);
+	cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLOCK_SOURCE_WATCHDOG |
+		       CLOCK_SOURCE_DEFERRED_UNSTABLE);
 	cs->flags |= CLOCK_SOURCE_UNSTABLE;
 
 	/*
@@ -215,6 +216,11 @@ static void __clocksource_unstable(struct clocksource *cs)
 
 	if (cs->mark_unstable)
 		cs->mark_unstable(cs);
+}
+
+static void __clocksource_deferred_unstable(struct clocksource *cs)
+{
+	cs->flags |= CLOCK_SOURCE_DEFERRED_UNSTABLE;
 
 	/* kick clocksource_watchdog_kthread() */
 	if (finished_booting)
@@ -236,7 +242,7 @@ void clocksource_mark_unstable(struct clocksource *cs)
 	if (!(cs->flags & CLOCK_SOURCE_UNSTABLE)) {
 		if (!list_empty(&cs->list) && list_empty(&cs->wd_list))
 			list_add(&cs->wd_list, &watchdog_list);
-		__clocksource_unstable(cs);
+		__clocksource_deferred_unstable(cs);
 	}
 	spin_unlock_irqrestore(&watchdog_lock, flags);
 }
@@ -453,7 +459,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 
 		if (read_ret == WD_READ_UNSTABLE) {
 			/* Clock readout unreliable, so give it up. */
-			__clocksource_unstable(cs);
+			__clocksource_deferred_unstable(cs);
 			continue;
 		}
 
@@ -538,7 +544,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 				pr_warn("                      '%s' (not '%s') is current clocksource.\n", curr_clocksource->name, cs->name);
 			else
 				pr_warn("                      No current clocksource.\n");
-			__clocksource_unstable(cs);
+			__clocksource_deferred_unstable(cs);
 			continue;
 		}
 
@@ -703,6 +709,8 @@ static int __clocksource_watchdog_kthread(void)
 
 	spin_lock_irqsave(&watchdog_lock, flags);
 	list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list) {
+		if (cs->flags & CLOCK_SOURCE_DEFERRED_UNSTABLE)
+			__clocksource_unstable(cs);
 		if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
 			list_del_init(&cs->wd_list);
 			clocksource_change_rating(cs, 0);

Paul!

On Wed, Mar 05 2025 at 09:43, Paul E. McKenney wrote:

I'm pretty sure, that I pointed this out to you before:

   "[PATCH subsys] Short description"

is not a valid subject line.

The canonical and documented form is:

   [PATCH] subsys: Short description

Can you please stop making your own rules and thereby breaking the
automated workflow of people so they have to manually fix up your stuff?

Thanks,

        tglx

On Thu, Mar 06, 2025 at 09:25:19AM +0100, Thomas Gleixner wrote:
> Paul!
> 
> On Wed, Mar 05 2025 at 09:43, Paul E. McKenney wrote:
> 
> I'm pretty sure, that I pointed this out to you before:
> 
>    "[PATCH subsys] Short description"
> 
> is not a valid subject line.
> 
> The canonical and documented form is:
> 
>    [PATCH] subsys: Short description
> 
> Can you please stop making your own rules and thereby breaking the
> automated workflow of people so they have to manually fix up your stuff?

Apologies!

I do not recall being chastised on this particular issue, but I clearly
do need to reread submittingpatches.rst more often.

I have adjusted my scripts for this particular issue and will resend v2
of this patch.

							Thanx, Paul