The commit a1ed5b0c
(klist: don't iterate over deleted entries) introduces use of the
low bit in a pointer to indicate if the knode is dead or not,
assuming that this bit is always free.

This is not true for all architectures, CRIS for example may align data
on byte borders.

The result is a bunch of warnings on bootup, devices not being
added correctly etc, reported by Hinko Kocevar <hinko.kocevar@cetrtapot.si>:

------------[ cut here ]------------
WARNING: at lib/klist.c:62 ()
Modules linked in:

Stack from c1fe1cf0:
       c01cc7f4 c1fe1d11 c000eb4e c000e4de 00000000 00000000 c1f4f78f c1f50c2d
       c01d008c c1fdd1a0 c1fdd1a0 c1fe1d38 c0192954 c1fe0000 00000000 c1fe1dc0
       00000002 7fffffff c1fe1da8 c0192d50 c1fe1dc0 00000002 7fffffff c1ff9fcc
Call Trace: [<c000eb4e>] [<c000e4de>] [<c0192954>] [<c0192d50>] [<c001d49e>] [<c000b688>] [<c0192a3c>]
       [<c000b63e>] [<c000b63e>] [<c001a542>] [<c00b55b0>] [<c00411c0>] [<c00b559c>] [<c01918e6>] [<c0191988>]
       [<c01919d0>] [<c00cd9c8>] [<c00cdd6a>] [<c0034178>] [<c000409a>] [<c0015576>] [<c0029130>] [<c0029078>]
       [<c0029170>] [<c0012336>] [<c00b4076>] [<c00b4770>] [<c006d6e4>] [<c006d974>] [<c006dca0>] [<c0028d6c>]
       [<c0028e12>] [<c0006424>] <4>---[ end trace 4eaa2a86a8e2da22 ]---
------------[ cut here ]------------
Repeat ad nauseam.

Wed, Jan 14, 2009 at 12:11:32AM +0100, Bastien ROUCARIES wrote:
> Perhaps using a pointerhackalign trick on this structure where
> #define pointerhackalign(x) __attribute__ ((aligned (x)))
> and declare
> struct klist_node {
> ...
> }  pointerhackalign(2);
>
> Because  __attribute__ ((aligned (x))) could only increase alignment
> it will safe to do that and serve as documentation purpose :)

That works, but we need to do it not for the struct klist_node,
but for the struct we insert into the void * in klist_node,
which is struct klist.

Reported-by: Hinko Kocevar <hinko.kocevar@cetrtapot.si
Cc: Bastien ROUCARIES <roucaries.bastien@gmail.com>
Signed-off-by: NJesper Nilsson <jesper.nilsson@axis.com>
Cc: stable <stable@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Removing the completion from klist_node reduces its size from 64 bytes
to 28 on x86-64.  To maintain the semantics of klist_remove(), we add
a single list of klist nodes which are pending deletion and scan them.
Signed-off-by: NMatthew Wilcox <willy@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

A klist entry is kept on the list till all its current iterations are
finished; however, a new iteration after deletion also iterates over
deleted entries as long as their reference count stays above zero.
This causes problems for cases where there are users which iterate
over the list while synchronized against list manipulations and
natuarally expect already deleted entries to not show up during
iteration.

This patch implements dead flag which gets set on deletion so that
iteration can skip already deleted entries.  The dead flag piggy backs
on the lowest bit of knode->n_klist and only visible to klist
implementation proper.

While at it, drop klist_iter->i_head as it's redundant and doesn't
offer anything in semantics or performance wise as klist_iter->i_klist
is dereferenced on every iteration anyway.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

Finally clean up the odd spacing in these files.
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Add klist_add_after() and klist_add_before() which puts a new node
after and before an existing node, respectively.  This is useful for
callers which need to keep klist ordered.  Note that synchronizing
between simultaneous additions for ordering is the caller's
responsibility.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

klist is missing static initializers and definition helper.  Add them.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

The problem is that klists claim to provide semantics for safe traversal of
lists which are being modified.  The failure case is when traversal of a
list causes element removal (a fairly common case).  The issue is that
although the list node is refcounted, if it is embedded in an object (which
is universally the case), then the object will be freed regardless of the
klist refcount leading to slab corruption because the klist iterator refers
to the prior element to get the next.

The solution is to make the klist take and release references to the
embedding object meaning that the embedding object won't be released until
the list relinquishes the reference to it.

(akpm: fast-track this because it's needed for the 2.6.13 scsi merge)
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

at the moment, the list_head semantics are

list_add(node, head)

whereas current klist semantics are

klist_add(head, node)

This is bound to cause confusion, and since klist is the newcomer, it
should follow the list_head semantics.

I also added missing include guards to klist.h
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Signed-off-by: NPatrick Mochel <mochel@digitalimplant.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

diff -Nru a/include/linux/klist.h b/include/linux/klist.h

This klist interface provides a couple of structures that wrap around
struct list_head to provide explicit list "head" (struct klist) and
list "node" (struct klist_node) objects. For struct klist, a spinlock
is included that protects access to the actual list itself. struct
klist_node provides a pointer to the klist that owns it and a kref
reference count that indicates the number of current users of that node
in the list.

The entire point is to provide an interface for iterating over a list
that is safe and allows for modification of the list during the
iteration (e.g. insertion and removal), including modification of the
current node on the list.

It works using a 3rd object type - struct klist_iter - that is declared
and initialized before an iteration. klist_next() is used to acquire the
next element in the list. It returns NULL if there are no more items.
This klist interface provides a couple of structures that wrap around
struct list_head to provide explicit list "head" (struct klist) and
list "node" (struct klist_node) objects. For struct klist, a spinlock
is included that protects access to the actual list itself. struct
klist_node provides a pointer to the klist that owns it and a kref
reference count that indicates the number of current users of that node
in the list.

The entire point is to provide an interface for iterating over a list
that is safe and allows for modification of the list during the
iteration (e.g. insertion and removal), including modification of the
current node on the list.

It works using a 3rd object type - struct klist_iter - that is declared
and initialized before an iteration. klist_next() is used to acquire the
next element in the list. It returns NULL if there are no more items.
Internally, that routine takes the klist's lock, decrements the reference
count of the previous klist_node and increments the count of the next
klist_node. It then drops the lock and returns.

There are primitives for adding and removing nodes to/from a klist.
When deleting, klist_del() will simply decrement the reference count.
Only when the count goes to 0 is the node removed from the list.
klist_remove() will try to delete the node from the list and block
until it is actually removed. This is useful for objects (like devices)
that have been removed from the system and must be freed (but must wait
until all accessors have finished).

Internally, that routine takes the klist's lock, decrements the reference
count of the previous klist_node and increments the count of the next
klist_node. It then drops the lock and returns.

There are primitives for adding and removing nodes to/from a klist.
When deleting, klist_del() will simply decrement the reference count.
Only when the count goes to 0 is the node removed from the list.
klist_remove() will try to delete the node from the list and block
until it is actually removed. This is useful for objects (like devices)
that have been removed from the system and must be freed (but must wait
until all accessors have finished).
Signed-off-by: NPatrick Mochel <mochel@digitalimplant.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

diff -Nru a/include/linux/klist.h b/include/linux/klist.h