Re: 2.6.24-rc6: possible recursive locking detected

Cyrill Gorcunov
2008-01-06T21:54:22Z

[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 -