futex: Relax the rcu_assign_pointer() assignment of mm->futex_phash in futex_mm_init()

The following commit added an rcu_assign_pointer() assignment to
futex_mm_init() in <linux/futex.h>:

  bd54df5ea7 ("futex: Allow to resize the private local hash")

Which breaks the build on older compilers (gcc-9, x86-64 defconfig):

   CC      io_uring/futex.o
   In file included from ./arch/x86/include/generated/asm/rwonce.h:1,
                    from ./include/linux/compiler.h:390,
                    from ./include/linux/array_size.h:5,
                    from ./include/linux/kernel.h:16,
                    from io_uring/futex.c:2:
   ./include/linux/futex.h: In function 'futex_mm_init':
   ./include/linux/rcupdate.h:555:36: error: dereferencing pointer to incomplete type 'struct futex_private_hash'

The problem is that this variant of rcu_assign_pointer() wants to
know the full type of 'struct futex_private_hash', which type
is local to futex.c:

   kernel/futex/core.c:struct futex_private_hash {

There are a couple of mechanical solutions for this bug:

  - we can uninline futex_mm_init() and move it into futex/core.c

  - or we can share the structure definition with kernel/fork.c.

But both of these solutions have disadvantages: the first one adds
runtime overhead, while the second one dis-encapsulates private
futex types.

A third solution, implemented by this patch, is to just initialize
mm->futex_phash with NULL like the patch below, it's not like this
new MM's ->futex_phash can be observed externally until the task
is inserted into the task list, which guarantees full store ordering.

The relaxation of this initialization might also give a tiny speedup
on certain platforms.

Fixes: bd54df5ea7 ("futex: Allow to resize the private local hash")
Signed-off-by: Ingo Molnar <[email protected]>
Cc: André Almeida <[email protected]>
Cc: Darren Hart <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Waiman Long <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

include/linux/futex.h

@@ -88,7 +88,14 @@ void futex_hash_free(struct mm_struct *mm);
 
 static inline void futex_mm_init(struct mm_struct *mm)
 {
-	rcu_assign_pointer(mm->futex_phash, NULL);
+	/*
+	 * No need for rcu_assign_pointer() here, as we can rely on
+	 * tasklist_lock write-ordering in copy_process(), before
+	 * the task's MM becomes visible and the ->futex_phash
+	 * becomes externally observable:
+	 */
+	mm->futex_phash = NULL;
+
 	mutex_init(&mm->futex_hash_lock);
 }