[IA64] ptrace - find memory sharers on children list

In arch/ia64/kernel/ptrace.c there is a test for a peek or poke of a
register image (in register backing storage).
The test can be unnecessarily long (and occurs while holding the tasklist_lock).
Especially long on a large system with thousands of active tasks.

The ptrace caller (presumably a debugger) specifies the pid of
its target and an address to peek or poke.  But the debugger could be
attached to several tasks.
The idea of find_thread_for_addr() is to find whether the target address
is in the RBS for any of those tasks.

Currently it searches the thread-list of the target pid.  If that search
does not find a match, and the shared mm-struct's user count indicates
that there are other tasks sharing this address space (a rare occurrence),
a search is made of all the tasks in the system.

Another approach can drastically shorten this procedure.
It depends upon the fact that in order to peek or poke from/to any task,
the debugger must first attach to that task.  And when it does, the
attached task is made a child of the debugger (is chained to its children list).

Therefore we can search just the debugger's children list.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

1 files changed, 10 insertions, 16 deletions

diff --git a/arch/ia64/kernel/ptrace.c b/arch/ia64/kernel/ptrace.c
index bbb8bc7c0552..9a9c1bd01dbc 100644
--- a/arch/ia64/kernel/ptrace.c
+++ b/arch/ia64/kernel/ptrace.c
@@ -589,6 +589,7 @@ find_thread_for_addr (struct task_struct *child, unsigned long addr)
 {
         struct task_struct *g, *p;
         struct mm_struct *mm;
+        struct list_head *this, *next;
         int mm_users;
 
         if (!(mm = get_task_mm(child)))
@@ -600,28 +601,21 @@ find_thread_for_addr (struct task_struct *child, unsigned long addr)
                 goto out;               /* not multi-threaded */
 
         /*
-         * First, traverse the child's thread-list.  Good for scalability with
-         * NPTL-threads.
+         * Traverse the current process' children list.  Every task that
+         * one attaches to becomes a child.  And it is only attached children
+         * of the debugger that are of interest (ptrace_check_attach checks
+         * for this).
          */
-        p = child;
-        do {
-                if (thread_matches(p, addr)) {
-                        child = p;
-                        goto out;
-                }
-                if (mm_users-- <= 1)
-                        goto out;
-        } while ((p = next_thread(p)) != child);
-
-        do_each_thread(g, p) {
-                if (child->mm != mm)
+        list_for_each_safe(this, next, &current->children) {
+                p = list_entry(this, struct task_struct, sibling);
+                if (p->mm != mm)
                         continue;
-
                 if (thread_matches(p, addr)) {
                         child = p;
                         goto out;
                 }
-        } while_each_thread(g, p);
+        }
+
   out:
         mmput(mm);
         return child;