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From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Davide Libenzi <davidel@xmailserver.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ingo Molnar <mingo@elte.hu>, "Rafael J. Wysocki" <rjw@sisk.pl>,
	Christian Kujau <lists@nerdbynature.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	jfs-discussion@lists.sourceforge.net,
	Johannes Berg <johannes@sipsolutions.net>,
	Oleg Nesterov <oleg@tv-sign.ru>
Subject: Re: 2.6.24-rc6: possible recursive locking detected
Date: Mon, 7 Jan 2008 00:53:56 +0300	[thread overview]
Message-ID: <20080106215356.GB32187@cvg> (raw)
In-Reply-To: <20080106214442.GA32187@cvg>

[Cyrill Gorcunov - Mon, Jan 07, 2008 at 12:44:42AM +0300]
| [Davide Libenzi - Sat, Jan 05, 2008 at 01:35:25PM -0800]
| | On Sat, 5 Jan 2008, Peter Zijlstra wrote:
| | 
| [...snip...] 
| | I remember I talked with Arjan about this time ago. Basically, since 1) 
| | you can drop an epoll fd inside another epoll fd 2) callback-based wakeups 
| | are used, you can see a wake_up() from inside another wake_up(), but they 
| | will never refer to the same lock instance.
| | Think about:
| | 
| | 	dfd = socket(...);
| | 	efd1 = epoll_create();
| | 	efd2 = epoll_create();
| | 	epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
| | 	epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
| | 
| | When a packet arrives to the device underneath "dfd", the net code will 
| | issue a wake_up() on its poll wake list. Epoll (efd1) has installed a 
| | callback wakeup entry on that queue, and the wake_up() performed by the 
| | "dfd" net code will end up in ep_poll_callback(). At this point epoll 
| | (efd1) notices that it may have some event ready, so it needs to wake up 
| | the waiters on its poll wait list (efd2). So it calls ep_poll_safewake() 
| | that ends up in another wake_up(), after having checked about the 
| | recursion constraints. That are, no more than EP_MAX_POLLWAKE_NESTS, to 
| | avoid stack blasting. Never hit the same queue, to avoid loops like:
| | 
| | 	epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
| | 	epoll_ctl(efd3, EPOLL_CTL_ADD, efd2, ...);
| | 	epoll_ctl(efd4, EPOLL_CTL_ADD, efd3, ...);
| | 	epoll_ctl(efd1, EPOLL_CTL_ADD, efd4, ...);
| | 
| | The code "if (tncur->wq == wq || ..." prevents re-entering the same 
| | queue/lock.
| | I don't know how the lockdep code works, so I can't say about 
| | wake_up_nested(). Although I have a feeling is not enough in this case.
| | A solution may be to move the call to ep_poll_safewake() (that'd become a 
| | simple wake_up()) inside a tasklet or whatever is today trendy for delayed 
| | work. But his kinda scares me to be honest, since epoll has already a 
| | bunch of places where it could be asynchronously hit (plus performance 
| | regression will need to be verified).
| | 
| | 
| | 
| | - Davide
| | 
| | 
| 
| it's quite possible that i'm wrong but just interested...
| why in ep_poll_safewake() the assignment
| 
| 	struct list_head *lsthead = &psw->wake_task_list;
| 
| is not protected by spinlock?
| 
| 		- Cyrill -

it was a completely stupid question... please drop ;)

		- Cyrill -

  reply	other threads:[~2008-01-06 21:54 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-03 22:58 Christian Kujau
2008-01-03 23:06 ` Rafael J. Wysocki
2008-01-04  8:30   ` Ingo Molnar
2008-01-05  7:12     ` Herbert Xu
2008-01-05 16:53       ` Peter Zijlstra
2008-01-05 17:01         ` Peter Zijlstra
2008-01-05 21:35           ` Davide Libenzi
2008-01-06  0:20             ` Christian Kujau
2008-01-07 21:35               ` Davide Libenzi
2008-01-06 21:44             ` Cyrill Gorcunov
2008-01-06 21:53               ` Cyrill Gorcunov [this message]
2008-01-07 17:22           ` Oleg Nesterov
2008-01-07 17:49             ` Oleg Nesterov
2008-01-13 16:32               ` Peter Zijlstra
2008-01-14 21:27                 ` Oleg Nesterov
2008-01-30 10:34                   ` hrtimers and lockdep (was: Re: 2.6.24-rc6: possible recursive locking detected) Peter Zijlstra
2008-01-30 17:36                     ` Thomas Gleixner

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