fs/epoll: make nesting accounting safe for -rt kernel

fs/epoll: make nesting accounting safe for -rt kernel

Davidlohr Bueso pointed out that when CONFIG_DEBUG_LOCK_ALLOC is set
ep_poll_safewake() can take several non-raw spinlocks after disabling
interrupts.  Since a spinlock can block in the -rt kernel, we can't take a
spinlock after disabling interrupts.  So let's re-work how we determine
the nesting level such that it plays nicely with the -rt kernel.

Let's introduce a 'nests' field in struct eventpoll that records the
current nesting level during ep_poll_callback().  Then, if we nest again
we can find the previous struct eventpoll that we were called from and
increase our count by 1.  The 'nests' field is protected by
ep->poll_wait.lock.

I've also moved the visited field to reduce the size of struct eventpoll
from 184 bytes to 176 bytes on x86_64 for !CONFIG_DEBUG_LOCK_ALLOC, which
is typical for a production config.

Reported-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Jason Baron <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Cc: Roman Penyaev <[email protected]>
Cc: Eric Wong <[email protected]>
Cc: Al Viro <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>