timekeeping: Register default clocksource before taking tk_core.lock

commit 8fa3082 upstream.

Commit f24df84 ("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: f24df84 ("time/jiffies: Register jiffies clocksource before usage")
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reported-by: Breno Leitao <leitao@debian.org>
Reported-by: Oleg Nesterov <oleg@redhat.com>
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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: NTMan

kernel/time/timekeeping.c

@@ -1825,13 +1825,14 @@ void __init timekeeping_init(void)
 	 */
 	wall_to_mono = timespec64_sub(boot_offset, wall_time);
 
+	clock = clocksource_default_clock();
+	if (clock->enable)
+		clock->enable(clock);
+
 	guard(raw_spinlock_irqsave)(&tk_core.lock);
 
 	ntp_init();
 
-	clock = clocksource_default_clock();
-	if (clock->enable)
-		clock->enable(clock);
 	tk_setup_internals(tks, clock);
 
 	tk_set_xtime(tks, &wall_time);