once: fix race by moving DO_ONCE to separate section

The commit c2c60ea ("once: use __section(".data.once")") moved
DO_ONCE's ___done variable to .data.once section, which conflicts with
DO_ONCE_LITE() that also uses the same section.

This creates a race condition when clear_warn_once is used:

Thread 1 (DO_ONCE)             Thread 2 (DO_ONCE)
__do_once_start
    read ___done (false)
    acquire once_lock
execute func
__do_once_done
    write ___done (true)      __do_once_start
    release once_lock             // Thread 3 clear_warn_once reset ___done
                                  read ___done (false)
                                  acquire once_lock
                              execute func
schedule once_work            __do_once_done
once_deferred: OK             write ___done (true)
static_branch_disable         release once_lock
                              schedule once_work
                              once_deferred:
                                  BUG_ON(!static_key_enabled)

DO_ONCE_LITE() in once_lite.h is used by WARN_ON_ONCE() and other warning
macros. Keep its ___done flag in the .data..once section and allow resetting
by clear_warn_once, as originally intended.

In contrast, DO_ONCE() is used for functions like get_random_once() and
relies on its ___done flag for internal synchronization. We should not reset
DO_ONCE() by clear_warn_once.

Fix it by isolating DO_ONCE's ___done into a separate .data..do_once section,
shielding it from clear_warn_once.

Fixes: c2c60ea ("once: use __section(".data.once")")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Author: Qi Xi

include/asm-generic/vmlinux.lds.h

@@ -361,6 +361,7 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 	__start_once = .;						\
 	*(.data..once)							\
 	__end_once = .;							\
+	*(.data..do_once)						\
 	STRUCT_ALIGN();							\
 	*(__tracepoints)						\
 	/* implement dynamic printk debug */				\

include/linux/once.h

@@ -46,7 +46,7 @@ void __do_once_sleepable_done(bool *done, struct static_key_true *once_key,
 #define DO_ONCE(func, ...)						     \
 	({								     \
 		bool ___ret = false;					     \
-		static bool __section(".data..once") ___done = false;	     \
+		static bool __section(".data..do_once") ___done = false;     \
 		static DEFINE_STATIC_KEY_TRUE(___once_key);		     \
 		if (static_branch_unlikely(&___once_key)) {		     \
 			unsigned long ___flags;				     \
@@ -64,7 +64,7 @@ void __do_once_sleepable_done(bool *done, struct static_key_true *once_key,
 #define DO_ONCE_SLEEPABLE(func, ...)						\
 	({									\
 		bool ___ret = false;						\
-		static bool __section(".data..once") ___done = false;		\
+		static bool __section(".data..do_once") ___done = false;	\
 		static DEFINE_STATIC_KEY_TRUE(___once_key);			\
 		if (static_branch_unlikely(&___once_key)) {			\
 			___ret = __do_once_sleepable_start(&___done);		\