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From: John Stultz <john.stultz@linaro.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: LKML <linux-kernel@vger.kernel.org>, "Ted Ts'o" <tytso@mit.edu>,
	David Rientjes <rientjes@google.com>,
	Dave Hansen <dave@linux.vnet.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access
Date: Mon, 16 May 2011 13:34:54 -0700	[thread overview]
Message-ID: <1305578094.2915.53.camel@work-vm> (raw)
In-Reply-To: <BANLkTin_MitzRUkWToj055AuAPdMC9msXQ@mail.gmail.com>

On Sat, 2011-05-14 at 20:12 +0900, KOSAKI Motohiro wrote:
> >> Can you please explain why we should use seqlock? That said,
> >> we didn't use seqlock for /proc items. because, plenty seqlock
> >> write may makes readers busy wait. Then, if we don't have another
> >> protection, we give the local DoS attack way to attackers.
> >
> > So you're saying that heavy write contention can cause reader
> > starvation?
> 
> Yes.
> 
> >> task->comm is used for very fundamentally. then, I doubt we can
> >> assume write is enough rare. Why can't we use normal spinlock?
> >
> > I think writes are likely to be fairly rare. Tasks can only name
> > themselves or sibling threads, so I'm not sure I see the risk here.
> 
> reader starvation may cause another task's starvation if reader have
> an another lock.

So the risk is a thread rewriting its own comm over and over could
starve some other critical task trying to read the comm.

Ok. It makes it a little more costly, but fair enough.

thanks
-john





WARNING: multiple messages have this Message-ID (diff)
From: John Stultz <john.stultz@linaro.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ted Ts'o <tytso@mit.edu>,
	David Rientjes <rientjes@google.com>,
	Dave Hansen <dave@linux.vnet.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access
Date: Mon, 16 May 2011 13:34:54 -0700	[thread overview]
Message-ID: <1305578094.2915.53.camel@work-vm> (raw)
In-Reply-To: <BANLkTin_MitzRUkWToj055AuAPdMC9msXQ@mail.gmail.com>

On Sat, 2011-05-14 at 20:12 +0900, KOSAKI Motohiro wrote:
> >> Can you please explain why we should use seqlock? That said,
> >> we didn't use seqlock for /proc items. because, plenty seqlock
> >> write may makes readers busy wait. Then, if we don't have another
> >> protection, we give the local DoS attack way to attackers.
> >
> > So you're saying that heavy write contention can cause reader
> > starvation?
> 
> Yes.
> 
> >> task->comm is used for very fundamentally. then, I doubt we can
> >> assume write is enough rare. Why can't we use normal spinlock?
> >
> > I think writes are likely to be fairly rare. Tasks can only name
> > themselves or sibling threads, so I'm not sure I see the risk here.
> 
> reader starvation may cause another task's starvation if reader have
> an another lock.

So the risk is a thread rewriting its own comm over and over could
starve some other critical task trying to read the comm.

Ok. It makes it a little more costly, but fair enough.

thanks
-john




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  reply	other threads:[~2011-05-16 20:35 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-05-12 23:02 [PATCH 0/3] v3 Improve task->comm locking situation John Stultz
2011-05-12 23:02 ` John Stultz
2011-05-12 23:02 ` [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access John Stultz
2011-05-12 23:02   ` John Stultz
2011-05-13 11:13   ` KOSAKI Motohiro
2011-05-13 11:13     ` KOSAKI Motohiro
2011-05-13 18:27     ` John Stultz
2011-05-13 18:27       ` John Stultz
2011-05-14 11:12       ` KOSAKI Motohiro
2011-05-14 11:12         ` KOSAKI Motohiro
2011-05-16 20:34         ` John Stultz [this message]
2011-05-16 20:34           ` John Stultz
2011-05-12 23:02 ` [PATCH 2/3] printk: Add %ptc to safely print a task's comm John Stultz
2011-05-12 23:02   ` John Stultz
2011-05-12 23:02 ` [PATCH 3/3] checkpatch.pl: Add check for current->comm references John Stultz
2011-05-12 23:02   ` John Stultz
2011-05-13  6:33   ` Jiri Slaby
2011-05-13  6:33     ` Jiri Slaby
2011-05-13 11:02     ` Michal Nazarewicz
2011-05-13 11:02       ` Michal Nazarewicz
2011-05-16 21:23       ` David Rientjes
2011-05-16 21:23         ` David Rientjes
2011-05-13 11:13   ` KOSAKI Motohiro
2011-05-13 11:13     ` KOSAKI Motohiro
2011-05-13 18:28     ` John Stultz
2011-05-13 18:28       ` John Stultz
  -- strict thread matches above, loose matches on Subject: below --
2011-05-16 21:19 [PATCH 0/3] v4 Improve task->comm locking situation John Stultz
2011-05-16 21:19 ` [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access John Stultz
2011-05-16 21:19   ` John Stultz
2011-05-16 22:01   ` Jiri Slaby
2011-05-16 22:01     ` Jiri Slaby
2011-05-17  1:47     ` John Stultz
2011-05-17  1:47       ` John Stultz
2011-05-18  0:28   ` KOSAKI Motohiro
2011-05-18  0:28     ` KOSAKI Motohiro
2011-05-11  0:23 [RFC][PATCH 0/3] v2 Improve task->comm locking situation John Stultz
2011-05-11  0:23 ` [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access John Stultz
2011-05-11  0:23   ` John Stultz
2011-05-11 17:39   ` Andi Kleen
2011-05-11 17:39     ` Andi Kleen
2011-05-12 22:00   ` David Rientjes
2011-05-12 22:00     ` David Rientjes
2011-04-28  4:03 [RFC][PATCH 0/3] Improve task->comm locking situation John Stultz
2011-04-28  4:03 ` [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access John Stultz
2011-04-28  4:03   ` John Stultz

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