Commit de047c1b authored by Ravikiran G Thirumalai's avatar Ravikiran G Thirumalai Committed by Linus Torvalds
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[PATCH] avoid tasklist_lock at getrusage for multithreaded case too



Avoid taking tasklist_lock for at getrusage for the multithreaded case too.
We don't need to take the tasklist lock for thread traversal of a process
since Oleg's do-__unhash_process-under-siglock.patch and related work.
Signed-off-by: default avatarRavikiran Thirumalai <kiran@scalex86.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent c89681ed
......@@ -1860,23 +1860,20 @@ out:
* fields when reaping, so a sample either gets all the additions of a
* given child after it's reaped, or none so this sample is before reaping.
*
* tasklist_lock locking optimisation:
* If we are current and single threaded, we do not need to take the tasklist
* lock or the siglock. No one else can take our signal_struct away,
* no one else can reap the children to update signal->c* counters, and
* no one else can race with the signal-> fields.
* If we do not take the tasklist_lock, the signal-> fields could be read
* out of order while another thread was just exiting. So we place a
* read memory barrier when we avoid the lock. On the writer side,
* write memory barrier is implied in __exit_signal as __exit_signal releases
* the siglock spinlock after updating the signal-> fields.
*
* We don't really need the siglock when we access the non c* fields
* of the signal_struct (for RUSAGE_SELF) even in multithreaded
* case, since we take the tasklist lock for read and the non c* signal->
* fields are updated only in __exit_signal, which is called with
* tasklist_lock taken for write, hence these two threads cannot execute
* concurrently.
* Locking:
* We need to take the siglock for CHILDEREN, SELF and BOTH
* for the cases current multithreaded, non-current single threaded
* non-current multithreaded. Thread traversal is now safe with
* the siglock held.
* Strictly speaking, we donot need to take the siglock if we are current and
* single threaded, as no one else can take our signal_struct away, no one
* else can reap the children to update signal->c* counters, and no one else
* can race with the signal-> fields. If we do not take any lock, the
* signal-> fields could be read out of order while another thread was just
* exiting. So we should place a read memory barrier when we avoid the lock.
* On the writer side, write memory barrier is implied in __exit_signal
* as __exit_signal releases the siglock spinlock after updating the signal->
* fields. But we don't do this yet to keep things simple.
*
*/
......@@ -1885,35 +1882,25 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
struct task_struct *t;
unsigned long flags;
cputime_t utime, stime;
int need_lock = 0;
memset((char *) r, 0, sizeof *r);
utime = stime = cputime_zero;
if (p != current || !thread_group_empty(p))
need_lock = 1;
if (need_lock) {
read_lock(&tasklist_lock);
if (unlikely(!p->signal)) {
read_unlock(&tasklist_lock);
return;
}
} else
/* See locking comments above */
smp_rmb();
rcu_read_lock();
if (!lock_task_sighand(p, &flags)) {
rcu_read_unlock();
return;
}
switch (who) {
case RUSAGE_BOTH:
case RUSAGE_CHILDREN:
spin_lock_irqsave(&p->sighand->siglock, flags);
utime = p->signal->cutime;
stime = p->signal->cstime;
r->ru_nvcsw = p->signal->cnvcsw;
r->ru_nivcsw = p->signal->cnivcsw;
r->ru_minflt = p->signal->cmin_flt;
r->ru_majflt = p->signal->cmaj_flt;
spin_unlock_irqrestore(&p->sighand->siglock, flags);
if (who == RUSAGE_CHILDREN)
break;
......@@ -1941,8 +1928,9 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
BUG();
}
if (need_lock)
read_unlock(&tasklist_lock);
unlock_task_sighand(p, &flags);
rcu_read_unlock();
cputime_to_timeval(utime, &r->ru_utime);
cputime_to_timeval(stime, &r->ru_stime);
}
......
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