Re: [patch V5 10/12] futex: Convert to get/put_user_inline()

[Christophe Leroy]

Le 27/10/2025 à 09:44, Thomas Gleixner a écrit :

From: Thomas Gleixner <t...@linutronix.de>

Replace the open coded implementation with the new get/put_user_inline()
helpers. This might be replaced by a regular get/put_user(), but that needs
a proper performance evaluation.

No functional change intended

Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Darren Hart <dvh...@infradead.org>
Cc: Davidlohr Bueso <d...@stgolabs.net>
Cc: "André Almeida" <andrealm...@igalia.com>

Reviewed-by: Christophe Leroy <christophe.le...@csgroup.eu>

---
V5: Rename again and remove the helpers
V4: Rename once moar
V3: Adapt to scope changes
V2: Convert to scoped variant
---
  kernel/futex/core.c  |    4 +--
  kernel/futex/futex.h |   58 
++-------------------------------------------------
  2 files changed, 5 insertions(+), 57 deletions(-)
---
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -581,7 +581,7 @@ int get_futex_key(u32 __user *uaddr, uns
        if (flags & FLAGS_NUMA) {
                u32 __user *naddr = (void *)uaddr + size / 2;

- if (futex_get_value(&node, naddr))

+               if (get_user_inline(node, naddr))
                        return -EFAULT;

if ((node != FUTEX_NO_NODE) &&

@@ -601,7 +601,7 @@ int get_futex_key(u32 __user *uaddr, uns
                        node = numa_node_id();
                        node_updated = true;
                }
-               if (node_updated && futex_put_value(node, naddr))
+               if (node_updated && put_user_inline(node, naddr))
                        return -EFAULT;
        }

--- a/kernel/futex/futex.h

+++ b/kernel/futex/futex.h
@@ -281,63 +281,11 @@ static inline int futex_cmpxchg_value_lo
        return ret;
  }

-/*

- * This does a plain atomic user space read, and the user pointer has
- * already been verified earlier by get_futex_key() to be both aligned
- * and actually in user space, just like futex_atomic_cmpxchg_inatomic().
- *
- * We still want to avoid any speculation, and while __get_user() is
- * the traditional model for this, it's actually slower than doing
- * this manually these days.
- *
- * We could just have a per-architecture special function for it,
- * the same way we do futex_atomic_cmpxchg_inatomic(), but rather
- * than force everybody to do that, write it out long-hand using
- * the low-level user-access infrastructure.
- *
- * This looks a bit overkill, but generally just results in a couple
- * of instructions.
- */
-static __always_inline int futex_get_value(u32 *dest, u32 __user *from)
-{
-       u32 val;
-
-       if (can_do_masked_user_access())
-               from = masked_user_access_begin(from);
-       else if (!user_read_access_begin(from, sizeof(*from)))
-               return -EFAULT;
-       unsafe_get_user(val, from, Efault);
-       user_read_access_end();
-       *dest = val;
-       return 0;
-Efault:
-       user_read_access_end();
-       return -EFAULT;
-}
-
-static __always_inline int futex_put_value(u32 val, u32 __user *to)
-{
-       if (can_do_masked_user_access())
-               to = masked_user_access_begin(to);
-       else if (!user_write_access_begin(to, sizeof(*to)))
-               return -EFAULT;
-       unsafe_put_user(val, to, Efault);
-       user_write_access_end();
-       return 0;
-Efault:
-       user_write_access_end();
-       return -EFAULT;
-}
-
+/* Read from user memory with pagefaults disabled */
  static inline int futex_get_value_locked(u32 *dest, u32 __user *from)
  {
-       int ret;
-
-       pagefault_disable();
-       ret = futex_get_value(dest, from);
-       pagefault_enable();
-
-       return ret;
+       guard(pagefault)();
+       return get_user_inline(*dest, from);
  }

extern void __futex_unqueue(struct futex_q *q);