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[IA64] ptrace - find memory sharers on children list

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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>
This commit is contained in:
Cliff Wickman 2005-10-27 10:29:08 -05:00 committed by Tony Luck
parent 72ab373a56
commit 4ac0068f44
1 changed files with 10 additions and 16 deletions
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26
arch/ia64/kernel/ptrace.c
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@ -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;