Re: 2.6.24-rc6: possible recursive locking detected

[Cyrill Gorcunov <gorcunov@gmail.com>]

[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 -