Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 1 Apr 2010 17:37:56 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH -mm] proc: don't take ->siglock for /proc/pid/oom_adj |
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On 04/01, David Rientjes wrote: > > On Thu, 1 Apr 2010, Oleg Nesterov wrote: > > > > That doesn't work for depraceted_mode (sic), you'd need to test for > > > OOM_ADJUST_MIN and OOM_ADJUST_MAX in that case. > > > > Yes, probably "if (depraceted_mode)" should do more checks, I didn't try > > to verify that MIN/MAX are correctly converted. I showed this code to explain > > what I mean. > > > > Ok, please cc me on the patch, it will be good to get rid of the duplicate > code and remove oom_adj from struct signal_struct.
OK, great, will do tomorrow.
> Do we need ->siglock? Why can't we just do > > struct sighand_struct *sighand; > struct signal_struct *sig; > > rcu_read_lock(); > sighand = rcu_dereference(task->sighand); > if (!sighand) { > rcu_read_unlock(); > return; > } > sig = task->signal; > > ... load/store to sig ... > > rcu_read_unlock();
No.
Before signals-make-task_struct-signal-immutable-refcountable.patch (actually, series of patches), this can't work. ->signal is not protected by rcu, and ->sighand != NULL doesn't mean ->signal != NULL.
(yes, thread_group_cputime() is wrong too, but currently it is never called lockless).
After signals-make-task_struct-signal-immutable-refcountable.patch, we do not need any checks at all, it is always safe to use ->signal.
But. Unless we kill signal->oom_adj, we have another reason for ->siglock, we can't update both oom_adj and oom_score_adj atomically, and if we race with another thread they can be inconsistent wrt each other. Yes, oom_adj is not actually used, except we report it back to user-space, but still.
So, I am going to send 2 patches. The first one factors out the code in base.c and kills signal->oom_adj, the next one removes ->siglock.
Oleg.
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