linux/net
Hans Verkuil 626cf23660 poll: add poll_requested_events() and poll_does_not_wait() functions
In some cases the poll() implementation in a driver has to do different
things depending on the events the caller wants to poll for.  An example
is when a driver needs to start a DMA engine if the caller polls for
POLLIN, but doesn't want to do that if POLLIN is not requested but instead
only POLLOUT or POLLPRI is requested.  This is something that can happen
in the video4linux subsystem among others.

Unfortunately, the current epoll/poll/select implementation doesn't
provide that information reliably.  The poll_table_struct does have it: it
has a key field with the event mask.  But once a poll() call matches one
or more bits of that mask any following poll() calls are passed a NULL
poll_table pointer.

Also, the eventpoll implementation always left the key field at ~0 instead
of using the requested events mask.

This was changed in eventpoll.c so the key field now contains the actual
events that should be polled for as set by the caller.

The solution to the NULL poll_table pointer is to set the qproc field to
NULL in poll_table once poll() matches the events, not the poll_table
pointer itself.  That way drivers can obtain the mask through a new
poll_requested_events inline.

The poll_table_struct can still be NULL since some kernel code calls it
internally (netfs_state_poll() in ./drivers/staging/pohmelfs/netfs.h).  In
that case poll_requested_events() returns ~0 (i.e.  all events).

Very rarely drivers might want to know whether poll_wait will actually
wait.  If another earlier file descriptor in the set already matched the
events the caller wanted to wait for, then the kernel will return from the
select() call without waiting.  This might be useful information in order
to avoid doing expensive work.

A new helper function poll_does_not_wait() is added that drivers can use
to detect this situation.  This is now used in sock_poll_wait() in
include/net/sock.h.  This was the only place in the kernel that needed
this information.

Drivers should no longer access any of the poll_table internals, but use
the poll_requested_events() and poll_does_not_wait() access functions
instead.  In order to enforce that the poll_table fields are now prepended
with an underscore and a comment was added warning against using them
directly.

This required a change in unix_dgram_poll() in unix/af_unix.c which used
the key field to get the requested events.  It's been replaced by a call
to poll_requested_events().

For qproc it was especially important to change its name since the
behavior of that field changes with this patch since this function pointer
can now be NULL when that wasn't possible in the past.

Any driver accessing the qproc or key fields directly will now fail to compile.

Some notes regarding the correctness of this patch: the driver's poll()
function is called with a 'struct poll_table_struct *wait' argument.  This
pointer may or may not be NULL, drivers can never rely on it being one or
the other as that depends on whether or not an earlier file descriptor in
the select()'s fdset matched the requested events.

There are only three things a driver can do with the wait argument:

1) obtain the key field:

	events = wait ? wait->key : ~0;

   This will still work although it should be replaced with the new
   poll_requested_events() function (which does exactly the same).
   This will now even work better, since wait is no longer set to NULL
   unnecessarily.

2) use the qproc callback. This could be deadly since qproc can now be
   NULL. Renaming qproc should prevent this from happening. There are no
   kernel drivers that actually access this callback directly, BTW.

3) test whether wait == NULL to determine whether poll would return without
   waiting. This is no longer sufficient as the correct test is now
   wait == NULL || wait->_qproc == NULL.

   However, the worst that can happen here is a slight performance hit in
   the case where wait != NULL and wait->_qproc == NULL. In that case the
   driver will assume that poll_wait() will actually add the fd to the set
   of waiting file descriptors. Of course, poll_wait() will not do that
   since it tests for wait->_qproc. This will not break anything, though.

   There is only one place in the whole kernel where this happens
   (sock_poll_wait() in include/net/sock.h) and that code will be replaced
   by a call to poll_does_not_wait() in the next patch.

   Note that even if wait->_qproc != NULL drivers cannot rely on poll_wait()
   actually waiting. The next file descriptor from the set might match the
   event mask and thus any possible waits will never happen.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: David Miller <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-03-23 16:58:38 -07:00
..
9p virtio: rename virtqueue_add_buf_gfp to virtqueue_add_buf 2012-01-12 15:44:42 +10:30
802 net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
8021q vlan: static functions 2011-12-14 02:39:30 -05:00
appletalk net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
atm ppp: Replace uses of <linux/if_ppp.h> with <linux/ppp-ioctl.h> 2012-03-04 20:41:38 -05:00
ax25 ax25: avoid overflows in ax25_setsockopt() 2011-12-28 14:08:08 -05:00
batman-adv Merge tag 'batman-adv-for-davem' of git://git.open-mesh.org/linux-merge 2012-03-11 15:36:34 -07:00
bluetooth Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-03-20 21:04:47 -07:00
bridge Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-03-09 14:34:20 -08:00
caif caif: make zero a legal caif connetion id. 2012-03-11 15:38:16 -07:00
can can: remove references to berlios mailinglist 2011-10-17 19:22:46 -04:00
ceph Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client 2012-02-02 15:47:33 -08:00
core net: fix napi_reuse_skb() skb reserve 2012-03-21 16:52:09 -04:00
dcb net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
dccp dccp: fix bug in sequence number validation during connection setup 2012-03-03 09:02:52 -07:00
decnet decnet: remove unused variable from dn_output() 2012-02-05 17:44:55 -05:00
dns_resolver KEYS: Allow special keyrings to be cleared 2012-01-19 14:38:51 +11:00
dsa dsa: Move switch drivers to new directory drivers/net/dsa 2011-11-29 00:21:36 -05:00
econet net: Remove all uses of LL_ALLOCATED_SPACE 2011-11-18 14:37:09 -05:00
ethernet eth: reset addr_assign_type if eth_mac_addr() called 2012-02-13 00:46:41 -05:00
ieee802154 net/ieee802154/6lowpan.c: reuse eth_mac_addr() 2012-02-22 14:46:37 -05:00
ipv4 bonding: remove entries for master_ip and vlan_ip and query devices instead 2012-03-22 22:36:17 -04:00
ipv6 netfilter: remove forward module param confusion. 2012-03-22 22:36:17 -04:00
ipx net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
irda Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-03-20 21:04:47 -07:00
iucv Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux 2012-03-22 18:15:32 -07:00
key net: use IS_ENABLED(CONFIG_IPV6) 2011-12-11 18:25:16 -05:00
l2tp l2tp: enable automatic module loading for l2tp_ppp 2012-03-21 22:14:56 -04:00
lapb wan: make LAPB callbacks const 2011-09-16 19:20:20 -04:00
llc llc: Fix race condition in llc_ui_recvmsg 2012-01-24 15:33:19 -05:00
mac80211 Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-03-20 21:04:47 -07:00
netfilter Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-03-20 21:04:47 -07:00
netlabel netlabel: use GFP flags from caller instead of GFP_ATOMIC 2012-03-22 19:29:57 -04:00
netlink netlink: allow to pass data pointer to netlink_dump_start() callback 2012-02-26 14:10:44 -05:00
netrom netrom: avoid overflows in nr_setsockopt() 2011-12-28 14:08:08 -05:00
nfc NFC: NCI code identation fixes 2012-03-06 15:16:25 -05:00
openvswitch Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-03-09 14:34:20 -08:00
packet net: Add framework to allow sending packets with customized CRC. 2012-02-24 01:37:35 -08:00
phonet net: reintroduce missing rcu_assign_pointer() calls 2012-01-12 12:26:56 -08:00
rds RDS: use gfp flags from caller in conn_alloc() 2012-03-22 19:29:58 -04:00
rfkill Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless-next into for-davem 2012-01-05 10:13:24 -05:00
rose net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
rxrpc RxRPC: Fix kcalloc parameters swapped 2012-02-14 14:41:55 -05:00
sched Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-03-20 21:04:47 -07:00
sctp sctp: Export sctp_do_peeloff 2012-03-08 13:52:08 -08:00
sunrpc NFS client updates for Linux 3.4 2012-03-23 08:53:47 -07:00
tipc tipc: Optimize setting of immutable payload message header fields 2012-02-29 11:45:35 -05:00
unix poll: add poll_requested_events() and poll_does_not_wait() functions 2012-03-23 16:58:38 -07:00
wanrouter wanrouter: Remove kernel_lock annotations 2011-11-07 13:27:30 -05:00
wimax net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00
wireless cfg80211: clarify timestamp in cfg80211_inform_bss 2012-03-13 14:54:20 -04:00
x25 net:x25: use IS_ENABLED 2011-12-16 15:49:52 -05:00
xfrm xfrm: Access the replay notify functions via the registered callbacks 2012-03-22 19:29:58 -04:00
compat.c net: get rid of some pointless casts to sockaddr 2012-03-11 19:11:22 -07:00
Kconfig net: Add Open vSwitch kernel components. 2011-12-03 09:35:17 -08:00
Makefile net: Add Open vSwitch kernel components. 2011-12-03 09:35:17 -08:00
nonet.c
socket.c net: get rid of some pointless casts to sockaddr 2012-03-11 19:11:22 -07:00
sysctl_net.c net: Add export.h for EXPORT_SYMBOL/THIS_MODULE to non-modules 2011-10-31 19:30:30 -04:00