Commit 4ac0068f authored Oct 27, 2005 by Cliff Wickman Committed by Tony Luck Oct 27, 2005

[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>

parent 72ab373a

arch/ia64/kernel/ptrace.c

+10 −16

Original line number	Diff line number	Diff line
		@@ -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;

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