linux-kernel - Re: [PATCH] perf_event: fix loss of notification with multi-event sampling

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CABPqkBT+XT4Oq9p0aquJgFZy8DBVudamFU4jRZqmdLUDR-Cpqg@mail.gmail.com>
Date:	Tue, 22 Nov 2011 15:28:02 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH] perf_event: fix loss of notification with multi-event sampling

On Tue, Nov 22, 2011 at 3:13 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Tue, Nov 22, 2011 at 2:29 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Tue, 2011-11-22 at 14:15 +0100, Stephane Eranian wrote:
>>
>>> > Ah, could it be a race of poll()/wakeup() vs perf_event_set_output() ?
>>> >
>>> Are you saying that by dropping event->waitq in favor of event->rb->waitq
>>> we make this problem disappear due to rcu protections?
>>
>> Well, except..
>>
>>> Poll_wait() is a blocking call. It may wait on a stale waitq.  But that problem
>>> was probably already there. I am not clear as to what to do about that.
>>> in perf_set_output() you would need to wakeup from poll_wait() and then
>>> go back in with the new waitq.
>>
>> Right, the whole blocking thing is a problem, and the whole poll()
>> interface always makes my head hurt.
>>
>> If there was a go-sleep and wake-up side to poll we could do
>> ring_buffer_get()/put() and fix this problem, but I'm not finding a way
>> to make that happen quite yet.
>>
>>> Similarly, I am not clear as to what happens when you close an event for
>>> which you have  a waiter in poll_wait(). I assume you wakeup from it.
>>> But I don't see where that's implemented.
>>
>> Good point, yes we should do that.
>>
> I looked at how this is done for regular files: eventpoll_release(file);

Well, but's that called as part of __fput(), which is called before
fops->release().
So it should do it, unless you have to set some more flags in poll(),
i.e., more than
POLLIN.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/