epoll: prevent missed events on EPOLL_CTL_MOD

EPOLL_CTL_MOD sets the interest mask before calling f_op->poll() to
ensure events are not missed.  Since the modifications to the interest
mask are not protected by the same lock as ep_poll_callback, we need to
ensure the change is visible to other CPUs calling ep_poll_callback.

We also need to ensure f_op->poll() has an up-to-date view of past
events which occured before we modified the interest mask.  So this
barrier also pairs with the barrier in wq_has_sleeper().

This should guarantee either ep_poll_callback or f_op->poll() (or both)
will notice the readiness of a recently-ready/modified item.

This issue was encountered by Andreas Voellmy and Junchang(Jason) Wang in:
http://thread.gmane.org/gmane.linux.kernel/1408782/

Signed-off-by: Eric Wong <[email protected]>
Cc: Hans Verkuil <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Davide Libenzi <[email protected]>
Cc: Hans de Goede <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: David Miller <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andreas Voellmy <[email protected]>
Tested-by: "Junchang(Jason) Wang" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions