On 08/14, Rik van Riel wrote:
>
> @@ -288,18 +288,31 @@ void thread_group_cputime(struct task_struct *tsk,
> struct task_cputime *times)
> struct signal_struct *sig = tsk->signal;
> cputime_t utime, stime;
> struct task_struct *t;
> -
> - times->utime = sig->utime;
> - times->stime = sig->stime;
> - times->sum_exec_runtime = sig->sum_sched_runtime;
> + unsigned int seq, nextseq;
>
> rcu_read_lock();
> - for_each_thread(tsk, t) {
> - task_cputime(t, &utime, &stime);
> - times->utime += utime;
> - times->stime += stime;
> - times->sum_exec_runtime += task_sched_runtime(t);
> - }
> + /* Attempt a lockless read on the first round. */
> + nextseq = 0;
> + do {
> + seq = nextseq;
> + read_seqbegin_or_lock(&sig->stats_lock, &seq);
> + times->utime = sig->utime;
> + times->stime = sig->stime;
> + times->sum_exec_runtime = sig->sum_sched_runtime;
> +
> + for_each_thread(tsk, t) {
> + task_cputime(t, &utime, &stime);
> + times->utime += utime;
> + times->stime += stime;
> + times->sum_exec_runtime += task_sched_runtime(t);
> + }
> + /*
> + * If a writer is currently active, seq will be odd, and
> + * read_seqbegin_or_lock will take the lock.
> + */
> + nextseq = raw_read_seqcount(&sig->stats_lock.seqcount);
> + } while (need_seqretry(&sig->stats_lock, seq));
> + done_seqretry(&sig->stats_lock, seq);
> rcu_read_unlock();
> }
I still think this is not right. Let me quote my previous email,
> @@ -288,18 +288,31 @@ void thread_group_cputime(struct task_struct
*tsk, struct task_cputime *times)
> struct signal_struct *sig = tsk->signal;
> cputime_t utime, stime;
> struct task_struct *t;
> -
> - times->utime = sig->utime;
> - times->stime = sig->stime;
> - times->sum_exec_runtime = sig->sum_sched_runtime;
> + unsigned int seq, nextseq;
>
> rcu_read_lock();
Almost cosmetic nit, but afaics this patch expands the rcu critical
section
for no reason. We only need rcu_read_lock/unlock around
for_each_thread()
below.
> + nextseq = 0;
> + do {
> + seq = nextseq;
> + read_seqbegin_or_lock(&sig->stats_lock, &seq);
> + times->utime = sig->utime;
> + times->stime = sig->stime;
> + times->sum_exec_runtime = sig->sum_sched_runtime;
> +
> + for_each_thread(tsk, t) {
> + task_cputime(t, &utime, &stime);
> + times->utime += utime;
> + times->stime += stime;
> + times->sum_exec_runtime +=
task_sched_runtime(t);
> + }
> + /*
> + * If a writer is currently active, seq will be odd, and
> + * read_seqbegin_or_lock will take the lock.
> + */
> + nextseq = raw_read_seqcount(&sig->stats_lock.seqcount);
> + } while (need_seqretry(&sig->stats_lock, seq));
> + done_seqretry(&sig->stats_lock, seq);
Hmm. It seems that read_seqbegin_or_lock() is not used correctly. I
mean,
this code still can livelock in theory. Just suppose that anoter CPU
does
write_seqlock/write_sequnlock right after read_seqbegin_or_lock(). In
this
case "seq & 1" will be never true and thus "or_lock" will never happen.
IMO, this should be fixed. Either we should guarantee the forward
progress
or we should not play with read_seqbegin_or_lock() at all. This code
assumes
that sooner or later "nextseq = raw_read_seqcount()" should return the
odd
counter, but in theory this can never happen.
And if we want to fix this we do not need 2 counters, just we need to
set
"seq = 1" manually after need_seqretry() == T. Say, like
__dentry_path() does.
(but unlike __dentry_path() we do not need to worry about
rcu_read_unlock so
the code will be simpler).
I am wondering if it makes sense to introduce
bool read_seqretry_or_lock(const seqlock_t *sl, int *seq)
{
if (*seq & 1) {
read_sequnlock_excl(lock);
return false;
}
if (!read_seqretry(lock, *seq))
return false;
*seq = 1;
return true;
}
Or I missed your reply?
Oleg.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
