[PATCH] NOMMU: Make futexes work under NOMMU conditions

Make futexes work under NOMMU conditions.

This can be tested by running this in one shell:

#define SYSERROR(X, Y) \
do { if ((long)(X) == -1L) { perror(Y); exit(1); }} while(0)

int main()
{
int shmid, tmp, *f, n;

shmid = shmget(23, 4, IPC_CREAT|0666);
SYSERROR(shmid, "shmget");

f = shmat(shmid, NULL, 0);
SYSERROR(f, "shmat");

n = *f;
printf("WAIT: %p{%x}\n", f, n);
tmp = futex(f, FUTEX_WAIT, n, NULL, NULL, 0);
SYSERROR(tmp, "futex");
printf("WAITED: %d\n", tmp);

tmp = shmdt(f);
SYSERROR(tmp, "shmdt");

exit(0);
}

And then this in the other shell:

#define SYSERROR(X, Y) \
do { if ((long)(X) == -1L) { perror(Y); exit(1); }} while(0)

int main()
{
int shmid, tmp, *f;

shmid = shmget(23, 4, IPC_CREAT|0666);
SYSERROR(shmid, "shmget");

f = shmat(shmid, NULL, 0);
SYSERROR(f, "shmat");

(*f)++;
printf("WAKE: %p{%x}\n", f, *f);
tmp = futex(f, FUTEX_WAKE, 1, NULL, NULL, 0);
SYSERROR(tmp, "futex");
printf("WOKE: %d\n", tmp);

tmp = shmdt(f);
SYSERROR(tmp, "shmdt");

exit(0);
}

The first program will set up a SYSV IPC SHM segment and wait on a futex in it
for the number at the start to change. The program will increment that number
and wake the first program up. This leads to output of the form:

SHELL 1 SHELL 2
======================= =======================
# /dowait
WAIT: 0xc32ac000{0}
# /dowake
WAKE: 0xc32ac000{1}
WAITED: 0 WOKE: 1

Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/nommu-mmap.txt

···
 on ramfs or tmpfs mounts.
 
 
+=======
+FUTEXES
+=======
+
+Futexes are supported in NOMMU mode if the arch supports them.  An error will
+be given if an address passed to the futex system call lies outside the
+mappings made by a process or if the mapping in which the address lies does not
+support futexes (such as an I/O chardev mapping).
+
+
 =============
 NO-MMU MREMAP
 =============

mm/nommu.c

···
 EXPORT_SYMBOL(find_vma);
 
 /*
+ * find a VMA
+ * - we don't extend stack VMAs under NOMMU conditions
+ */
+struct vm_area_struct *find_extend_vma(struct mm_struct *mm, unsigned long addr)
+{
+	return find_vma(mm, addr);
+}
+
+/*
  * look up the first VMA exactly that exactly matches addr
  * - should be called with mm->mmap_sem at least held readlocked
  */
···
 
 struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 			unsigned int foll_flags)
-{
-	return NULL;
-}
-
-struct vm_area_struct *find_extend_vma(struct mm_struct *mm, unsigned long addr)
 {
 	return NULL;
 }