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Steven Rostedt 02edc6fc4d epoll: comment the funky #ifdef 
Looking for a bug in -rt, I stumbled across this code here from: commit
2dfa4eeab0 ("epoll keyed wakeups: teach epoll about hints coming with
the wakeup key"), specifically:

  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
                                      unsigned long events, int subclass)
  {
         unsigned long flags;

         spin_lock_irqsave_nested(&wqueue->lock, flags, subclass);
         wake_up_locked_poll(wqueue, events);
         spin_unlock_irqrestore(&wqueue->lock, flags);
  }
  #else
  static inline void ep_wake_up_nested(wait_queue_head_t *wqueue,
                                      unsigned long events, int subclass)
  {
         wake_up_poll(wqueue, events);
  }
  #endif

You change the function of ep_wake_up_nested() depending on whether
CONFIG_DEBUG_LOCK_ALLOC is set or not.  This looks awfully suspicious,
and there's no comment to explain why.  I initially thought that this
was trying to fool lockdep, and hiding a real bug.

Investigating it, I found the creation of wake_up_nested() (which no
longer exists) but was created for the sole purpose of epoll and its
strange wake ups, as explained in commit 0ccf831cbe ("lockdep:
annotate epoll")

Although the commit message says "annotate epoll" the change log is much
better at explaining what is happening than what is in the actual code.
Thus a comment is really necessary here.  And to save the time of other
developers from having to go trudging through the git logs trying to
figure out why this code exists.

I took parts of the change log and placed it into a comment above the
affected code.  This will make the description of what is happening more
visible to new developers that have to look at this code for the first
time.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: David Miller <davem@davemloft.net>
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 9p: make register_filesystem() the last failure exit 2012-03-20 21:29:45 -04:00
adfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
affs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
afs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
autofs4 autofs: set things up *before* registering fs type 2012-03-20 21:29:46 -04:00
befs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
bfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
btrfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
cachefiles switch touch_atime to struct path 2012-03-20 21:29:41 -04:00
ceph switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
cifs Merge git://git.samba.org/sfrench/cifs-2.6 2012-03-23 09:07:15 -07:00
coda switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
configfs make configfs_pin_fs() return root dentry on success 2012-03-20 21:29:48 -04:00
cramfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
debugfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
devpts Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
dlm dlm for 3.4 2012-03-21 13:54:22 -07:00
ecryptfs ecryptfs: make register_filesystem() the last potential failure exit 2012-03-20 21:29:49 -04:00
efs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
exofs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
exportfs
ext2 Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
ext3 switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
ext4 ext4: initialization of ext4_li_mtx needs to be done earlier 2012-03-20 22:05:02 -04:00
fat fat: switch to d_make_root() 2012-03-20 21:29:36 -04:00
freevxfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
fscache
fuse Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
gfs2 Merge git://git.kernel.org/pub/scm/linux/kernel/git/steve/gfs2-3.0-nmw 2012-03-21 18:00:03 -07:00
hfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
hfsplus hfsplus: add an ioctl to bless files 2012-03-20 21:29:53 -04:00
hostfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
hpfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
hppfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
hugetlbfs Merge branch 'akpm' (Andrew's patch-bomb) 2012-03-22 09:04:48 -07:00
isofs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
jbd Power management updates for 3.4 2012-03-21 10:15:51 -07:00
jbd2 Power management updates for 3.4 2012-03-21 10:15:51 -07:00
jffs2 Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
jfs jfs: mising cleanup on register_filesystem() failure 2012-03-20 21:29:48 -04:00
lockd SUNRPC/LOCKD: Fix build warnings when CONFIG_SUNRPC_DEBUG is undefined 2012-03-21 09:31:44 -04:00
logfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
minix Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
ncpfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
nfs NFS client updates for Linux 3.4 2012-03-23 08:53:47 -07:00
nfs_common
nfsd NFS client updates for Linux 3.4 2012-03-23 08:53:47 -07:00
nilfs2 Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
nls
notify fs/notify/notification.c: make subsys_initcall function static 2012-03-23 16:58:31 -07:00
ntfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
ocfs2 Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
omfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
openpromfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
proc Merge branch 'akpm' (Andrew's patch-bomb) 2012-03-22 09:04:48 -07:00
pstore One pstore patch 2012-03-23 09:24:07 -07:00
qnx4 qnx4: new helper - try_extent() 2012-03-20 21:29:52 -04:00
qnx6 fs: initial qnx6fs addition 2012-03-20 21:29:38 -04:00
quota Merge branch 'for-linus' of git://oss.sgi.com/xfs/xfs 2012-03-23 09:19:22 -07:00
ramfs tidy up after d_make_root() conversion 2012-03-20 21:29:37 -04:00
reiserfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
romfs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
squashfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
sysfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
sysv switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
ubifs - Improve error messages 2012-03-23 09:27:40 -07:00
udf Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
ufs switch open-coded instances of d_make_root() to new helper 2012-03-20 21:29:35 -04:00
xfs Merge branch 'for-linus' of git://oss.sgi.com/xfs/xfs 2012-03-23 09:19:22 -07:00
aio.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
anon_inodes.c anon_inodes: move allocation of anon_inode into ->mount() 2012-03-20 21:29:45 -04:00
attr.c
bad_inode.c
binfmt_aout.c take removal of PF_FORKNOEXEC to flush_old_exec() 2012-03-20 21:29:51 -04:00
binfmt_elf.c take removal of PF_FORKNOEXEC to flush_old_exec() 2012-03-20 21:29:51 -04:00
binfmt_elf_fdpic.c take removal of PF_FORKNOEXEC to flush_old_exec() 2012-03-20 21:29:51 -04:00
binfmt_em86.c __register_binfmt() made void 2012-03-20 21:29:46 -04:00
binfmt_flat.c take removal of PF_FORKNOEXEC to flush_old_exec() 2012-03-20 21:29:51 -04:00
binfmt_misc.c magic.h: move some FS magic numbers into magic.h 2012-03-23 16:58:31 -07:00
binfmt_script.c __register_binfmt() made void 2012-03-20 21:29:46 -04:00
binfmt_som.c take removal of PF_FORKNOEXEC to flush_old_exec() 2012-03-20 21:29:51 -04:00
bio-integrity.c fs: remove the second argument of k[un]map_atomic() 2012-03-20 21:48:21 +08:00
bio.c bio: don't overflow in bio_get_nr_vecs() 2012-02-08 22:07:18 +01:00
block_dev.c magic.h: move some FS magic numbers into magic.h 2012-03-23 16:58:31 -07:00
buffer.c
char_dev.c
compat.c vfs: fix compat_sys_stat() handling of overflows in st_nlink 2012-02-13 20:45:39 -05:00
compat_binfmt_elf.c
compat_ioctl.c ppp: Replace uses of <linux/if_ppp.h> with <linux/ppp-ioctl.h> 2012-03-04 20:41:38 -05:00
dcache.c fs: fix kernel-doc warnings in dcache.c 2012-03-22 15:49:18 -07:00
dcookies.c
direct-io.c Restore direct_io / truncate locking API 2012-02-23 15:56:21 -08:00
drop_caches.c
eventfd.c
eventpoll.c epoll: comment the funky #ifdef 2012-03-23 16:58:38 -07:00
exec.c Merge branch 'akpm' (Andrew's patch-bomb) 2012-03-22 09:04:48 -07:00
fcntl.c
fhandle.c
fifo.c
file.c
file_table.c vfs: drop_file_write_access() made static 2012-03-20 21:29:32 -04:00
filesystems.c
fs-writeback.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial 2012-03-20 21:12:50 -07:00
fs_struct.c vfs: take path_get_longterm() out of write_seqcount scope 2012-03-20 21:29:42 -04:00
generic_acl.c
inode.c trim includes in inode.c 2012-03-20 21:29:51 -04:00
internal.h
ioctl.c
ioprio.c block: strip out locking optimization in put_io_context() 2012-02-07 07:51:30 +01:00
Kconfig Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-03-21 13:36:41 -07:00
Kconfig.binfmt
libfs.c make simple_pin_fs() pass MS_KERNMOUNT - it's a kernel-internal one 2012-03-20 21:29:44 -04:00
locks.c
Makefile fs: initial qnx6fs addition 2012-03-20 21:29:38 -04:00
mbcache.c
mount.h
mpage.c
namei.c vfs: tidy up sparse warnings in fs/namei.c 2012-03-22 16:10:40 -07:00
namespace.c
no-block.c
open.c
pipe.c magic.h: move some FS magic numbers into magic.h 2012-03-23 16:58:31 -07:00
pnode.c
pnode.h
posix_acl.c
proc_namespace.c
read_write.c
read_write.h
readdir.c
select.c poll: add poll_requested_events() and poll_does_not_wait() functions 2012-03-23 16:58:38 -07:00
seq_file.c seq_file: fix mishandling of consecutive pread() invocations. 2012-03-21 17:54:54 -07:00
signalfd.c epoll: ep_unregister_pollwait() can use the freed pwq->whead 2012-02-24 11:42:50 -08:00
splice.c fs: remove the second argument of k[un]map_atomic() 2012-03-20 21:48:21 +08:00
stack.c
stat.c switch touch_atime to struct path 2012-03-20 21:29:41 -04:00
statfs.c
super.c Cleanups: rename of flush to invalidate, moving reporting of statistics 2012-03-22 19:52:47 -07:00
sync.c
timerfd.c
utimes.c
xattr.c
xattr_acl.c