The patch titled
avoid tasklist_lock at getrusage for multithreaded case too
has been removed from the -mm tree. Its filename is
avoid-tasklist_lock-at-getrusage-for-multithreaded-case-too.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
Subject: avoid tasklist_lock at getrusage for multithreaded case too
From: Ravikiran G Thirumalai <kiran@xxxxxxxxxxxx>
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: Ravikiran Thirumalai <kiran@xxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
---
kernel/sys.c | 56 +++++++++++++++++++------------------------------
1 file changed, 22 insertions(+), 34 deletions(-)
diff -puN kernel/sys.c~avoid-tasklist_lock-at-getrusage-for-multithreaded-case-too kernel/sys.c
--- a/kernel/sys.c~avoid-tasklist_lock-at-getrusage-for-multithreaded-case-too
+++ a/kernel/sys.c
@@ -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_stru
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_stru
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);
}
_
Patches currently in -mm which might be from kiran@xxxxxxxxxxxx are
origin.patch
percpu-counters-add-percpu_counter_exceeds.patch
slab-leaks3-default-y.patch
-
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