1. 10 Apr, 2012 8 commits
  2. 09 Apr, 2012 12 commits
  3. 27 Mar, 2012 1 commit
  4. 26 Mar, 2012 3 commits
  5. 25 Mar, 2012 1 commit
  6. 23 Mar, 2012 1 commit
    • Hans Verkuil's avatar
      poll: add poll_requested_events() and poll_does_not_wait() functions · 626cf236
      Hans Verkuil authored
      
      
      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: default avatarHans Verkuil <hans.verkuil@cisco.com>
      Reviewed-by: default avatarJonathan Corbet <corbet@lwn.net>
      Reviewed-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
      Cc: Davide Libenzi <davidel@xmailserver.org>
      Signed-off-by: default avatarHans 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: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      626cf236
  7. 22 Mar, 2012 11 commits
  8. 21 Mar, 2012 3 commits