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ptrace: optimize exit_ptrace() for the likely case

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exit_ptrace() takes tasklist_lock unconditionally.  We need this lock to
avoid the race with ptrace_traceme(), it acts as a barrier.

Change its caller, forget_original_parent(), to call exit_ptrace() under
tasklist_lock.  Change exit_ptrace() to drop and reacquire this lock if
needed.

This allows us to add the fastpath list_empty(ptraced) check.  In the
likely no-tracees case exit_ptrace() just returns and we avoid the lock()
+ unlock() sequence.

"Zhang, Yanmin" <yanmin_zhang@linux.intel.com> suggested to add this
check, and he reports that this change adds about 11% improvement in some
tests.

Suggested-and-tested-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2010-08-10 18:03:07 -07:00 committed by Linus Torvalds
parent 13d7e3a2db
commit c7e49c1488
2 changed files with 14 additions and 5 deletions
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7
kernel/exit.c
View file

@ -771,9 +771,12 @@ static void forget_original_parent(struct task_struct *father)
struct task_struct *p, *n, *reaper;
LIST_HEAD(dead_children);
exit_ptrace(father);
write_lock_irq(&tasklist_lock);
/*
* Note that exit_ptrace() and find_new_reaper() might
* drop tasklist_lock and reacquire it.
*/
exit_ptrace(father);
reaper = find_new_reaper(father);
list_for_each_entry_safe(p, n, &father->children, sibling) {

12
kernel/ptrace.c
View file

@ -324,26 +324,32 @@ int ptrace_detach(struct task_struct *child, unsigned int data)
}
/*
* Detach all tasks we were using ptrace on.
* Detach all tasks we were using ptrace on. Called with tasklist held
* for writing, and returns with it held too. But note it can release
* and reacquire the lock.
*/
void exit_ptrace(struct task_struct *tracer)
{
struct task_struct *p, *n;
LIST_HEAD(ptrace_dead);
write_lock_irq(&tasklist_lock);
if (likely(list_empty(&tracer->ptraced)))
return;
list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
if (__ptrace_detach(tracer, p))
list_add(&p->ptrace_entry, &ptrace_dead);
}
write_unlock_irq(&tasklist_lock);
write_unlock_irq(&tasklist_lock);
BUG_ON(!list_empty(&tracer->ptraced));
list_for_each_entry_safe(p, n, &ptrace_dead, ptrace_entry) {
list_del_init(&p->ptrace_entry);
release_task(p);
}
write_lock_irq(&tasklist_lock);
}
int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len)