Partial locking for mempolicy in task_struct

Currently the mempolicy in task_struct is a pointer to a structure containing
policy information for a task that is protected by no locks meaning that
only a currently executing task can reliably access that information.

The memory policies in the vm_area struct are protected by mmap_sem. This means
that memory policies from the vm_area_struct can be accessed after taking a readlock
on mmap_sem.

This patch extends the protection to all memory policies. Any writer to a memory
policy must take an mmap_sem writelock. Readers take the mmap_sem readlock. However,
this is not always possible since the policies are used in critical sections where
we cannot affort to take an mmap_sem lock.

We can therefore only protect against modifications and not exclude concurrent readers
unless the modification is executed in the context of the task.

The patch will therefore enable the reading of the memory policy from the task struct
by taking mmap_sem. It does *not* allow changing the policy in task_struct. The memory
policy must be changed from inside of the task_struct.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.13-rc5/mm/mempolicy.c
===================================================================
--- linux-2.6.13-rc5.orig/mm/mempolicy.c	2005-08-05 09:29:11.000000000 -0700
+++ linux-2.6.13-rc5/mm/mempolicy.c	2005-08-05 09:36:07.000000000 -0700
@@ -450,6 +450,7 @@ asmlinkage long sys_set_mempolicy(int mo
 
 	if (mode < 0 || mode > MPOL_MAX)
 		return -EINVAL;
+	down_write(&current->mm->mmap_sem);
 	err = get_nodes(&nodes, nmask, maxnode, mode);
 	if (err)
 		return err;
@@ -460,6 +461,7 @@ asmlinkage long sys_set_mempolicy(int mo
 	current->mempolicy = new;
 	if (new && new->policy == MPOL_INTERLEAVE)
 		current->il_next = first_node(new->v.nodes);
+	up_write(&current->mm->mmap_sem);
 	return 0;
 }
 
@@ -537,19 +539,21 @@ asmlinkage long sys_get_mempolicy(int __
 		return -EINVAL;
 	if (nmask != NULL && maxnode < MAX_NUMNODES)
 		return -EINVAL;
+	down_read(&mm->mmap_sem);
 	if (flags & MPOL_F_ADDR) {
-		down_read(&mm->mmap_sem);
 		vma = find_vma_intersection(mm, addr, addr + 1);
 		if (!vma) {
-			up_read(&mm->mmap_sem);
-			return -EFAULT;
+			err = -EFAULT;
+			goto out;
 		}
 		if (vma->vm_ops && vma->vm_ops->get_policy)
 			pol = vma->vm_ops->get_policy(vma, addr);
 		else
 			pol = vma->vm_policy;
-	} else if (addr)
-		return -EINVAL;
+	} else if (addr) {
+		err = -EINVAL;
+		goto out;
+	}
 
 	if (!pol)
 		pol = &default_policy;
@@ -570,10 +574,8 @@ asmlinkage long sys_get_mempolicy(int __
 	} else
 		pval = pol->policy;
 
-	if (vma) {
-		up_read(&current->mm->mmap_sem);
-		vma = NULL;
-	}
+	up_read(&mm->mmap_sem);
+	vma = NULL;
 
 	if (policy && put_user(pval, policy))
 		return -EFAULT;
@@ -586,8 +588,7 @@ asmlinkage long sys_get_mempolicy(int __
 	}
 
  out:
-	if (vma)
-		up_read(&current->mm->mmap_sem);
+	up_read(&mm->mmap_sem);
 	return err;
 }