[PATCH 6.12.y] timekeeping: Register default clocksource before taking tk_core.lock

[Thomas Gleixner <tglx@kernel.org>]

From: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

commit 8fa30821180a9a19e78e9f4df1c0ba710252801e upstream.

Commit f24df84cbe05 ("time/jiffies: Register jiffies clocksource before
usage") moved the jiffies clocksource registration into
clocksource_default_clock(), so that it is registered lazily on the first
call. __clocksource_register() acquires clocksource_mutex, but the first
caller is timekeeping_init(), which invokes clocksource_default_clock()
while holding tk_core.lock, a raw spinlock.

Acquiring a sleeping mutex while holding a raw spinlock is invalid.

The default clocksource only has to be registered before
tk_setup_internals() consumes its mult/shift/maxadj. Neither
clocksource_default_clock(), the ->enable() callback, nor the registration
itself need tk_core.lock, so fetch and enable the clock before acquiring
the lock. This preserves the "register before usage" ordering while
keeping clocksource_mutex out of the raw spinlock section.

clocksource_default_clock() has a second caller,
clocksource_done_booting(), which invokes it with clocksource_mutex already
held. That path avoids a recursive lock because timekeeping_init() has
already run and set cs_jiffies_registered, so the registration is skipped
there. This change does not alter that; it only fixes the invalid wait
context in timekeeping_init().

Fixes: f24df84cbe05 ("time/jiffies: Register jiffies clocksource before usage")
Reported-by: Breno Leitao <leitao@debian.org>
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260616070914.65818-1-mikhail.v.gavrilov@gmail.com
---
 kernel/time/timekeeping.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1685,13 +1685,14 @@ void __init timekeeping_init(void)
 	 */
 	wall_to_mono = timespec64_sub(boot_offset, wall_time);
 
+	clock = clocksource_default_clock();
+	if (clock->enable)
+		clock->enable(clock);
+
 	raw_spin_lock_irqsave(&timekeeper_lock, flags);
 	write_seqcount_begin(&tk_core.seq);
 	ntp_init();
 
-	clock = clocksource_default_clock();
-	if (clock->enable)
-		clock->enable(clock);
 	tk_setup_internals(tk, clock);
 
 	tk_set_xtime(tk, &wall_time);