futex: Restore one of the fast paths eliminated by 38d47c1b7075bd7ec3881141bb3629da58...

This improves futex performance until our user-space code is fixed to use
FUTEX_PRIVATE_FLAG for non-shared futexes.

Signed-off-by: Arve Hjønnevåg <arve@android.com>

diff --git a/kernel/futex.c b/kernel/futex.c
index 6a3a5fa1526d87d16b362e20d7d62e6809285c18..257db2460ab81542d61ba67443de70e0771c2c50 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -220,6 +220,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
        struct mm_struct *mm = current->mm;
        struct page *page;
        int err;
+       struct vm_area_struct *vma;
 
        /*
         * The futex address must be "naturally" aligned.
@@ -245,6 +246,37 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
                return 0;
        }
 
+       /*
+        * The futex is hashed differently depending on whether
+        * it's in a shared or private mapping.  So check vma first.
+        */
+       vma = find_extend_vma(mm, address);
+       if (unlikely(!vma))
+               return -EFAULT;
+
+       /*
+        * Permissions.
+        */
+       if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
+               return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;
+
+       /*
+        * Private mappings are handled in a simple way.
+        *
+        * NOTE: When userspace waits on a MAP_SHARED mapping, even if
+        * it's a read-only handle, it's expected that futexes attach to
+        * the object not the particular process.  Therefore we use
+        * VM_MAYSHARE here, not VM_SHARED which is restricted to shared
+        * mappings of _writable_ handles.
+        */
+       if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
+               key->both.offset |= FUT_OFF_MMSHARED; /* reference taken on mm */
+               key->private.mm = mm;
+               key->private.address = address;
+               get_futex_key_refs(key);
+               return 0;
+       }
+
 again:
        err = get_user_pages_fast(address, 1, 1, &page);
        if (err < 0)