A rare BUG_ON triggered in assoc_array_gc:
[3430308.818153] kernel BUG at lib/assoc_array.c:1609!
Which corresponded to the statement currently at line 1593 upstream:
BUG_ON(assoc_array_ptr_is_meta(p));
Using the data from the core dump, I was able to generate a userspace
reproducer[1] and determine the cause of the bug.
[1]: https://github.com/brenns10/kernel_stuff/tree/master/assoc_array_gc
After running the iterator on the entire branch, an internal tree node
looked like the following:
NODE (nr_leaves_on_branch: 3)
SLOT [0] NODE (2 leaves)
SLOT [1] NODE (1 leaf)
SLOT [2..f] NODE (empty)
In the userspace reproducer, the pr_devel output when compressing this
node was:
-- compress node 0x5607cc089380 --
free=0, leaves=0
[0] retain node 2/1 [nx 0]
[1] fold node 1/1 [nx 0]
[2] fold node 0/1 [nx 2]
[3] fold node 0/2 [nx 2]
[4] fold node 0/3 [nx 2]
[5] fold node 0/4 [nx 2]
[6] fold node 0/5 [nx 2]
[7] fold node 0/6 [nx 2]
[8] fold node 0/7 [nx 2]
[9] fold node 0/8 [nx 2]
[10] fold node 0/9 [nx 2]
[11] fold node 0/10 [nx 2]
[12] fold node 0/11 [nx 2]
[13] fold node 0/12 [nx 2]
[14] fold node 0/13 [nx 2]
[15] fold node 0/14 [nx 2]
after: 3
At slot 0, an internal node with 2 leaves could not be folded into the
node, because there was only one available slot (slot 0). Thus, the
internal node was retained. At slot 1, the node had one leaf, and was
able to be folded in successfully. The remaining nodes had no leaves,
and so were removed. By the end of the compression stage, there were 14
free slots, and only 3 leaf nodes. The tree was ascended and then its
parent node was compressed. When this node was seen, it could not be
folded, due to the internal node it contained.
The invariant for compression in this function is: whenever
nr_leaves_on_branch < ASSOC_ARRAY_FAN_OUT, the node should contain all
leaf nodes. The compression step currently cannot guarantee this, given
the corner case shown above.
To fix this issue, retry compression whenever we have retained a node,
and yet nr_leaves_on_branch < ASSOC_ARRAY_FAN_OUT. This second
compression will then allow the node in slot 1 to be folded in,
satisfying the invariant. Below is the output of the reproducer once the
fix is applied:
-- compress node 0x560e9c562380 --
free=0, leaves=0
[0] retain node 2/1 [nx 0]
[1] fold node 1/1 [nx 0]
[2] fold node 0/1 [nx 2]
[3] fold node 0/2 [nx 2]
[4] fold node 0/3 [nx 2]
[5] fold node 0/4 [nx 2]
[6] fold node 0/5 [nx 2]
[7] fold node 0/6 [nx 2]
[8] fold node 0/7 [nx 2]
[9] fold node 0/8 [nx 2]
[10] fold node 0/9 [nx 2]
[11] fold node 0/10 [nx 2]
[12] fold node 0/11 [nx 2]
[13] fold node 0/12 [nx 2]
[14] fold node 0/13 [nx 2]
[15] fold node 0/14 [nx 2]
internal nodes remain despite enough space, retrying
-- compress node 0x560e9c562380 --
free=14, leaves=1
[0] fold node 2/15 [nx 0]
after: 3
Changes
=======
DH:
- Use false instead of 0.
- Reorder the inserted lines in a couple of places to put retained before
next_slot.
ver #2)
- Fix typo in pr_devel, correct comparison to "<="
Fixes: 3cb989501c ("Add a generic associative array implementation.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: keyrings@vger.kernel.org
Link: https://lore.kernel.org/r/20220511225517.407935-1-stephen.s.brennan@oracle.com/ # v1
Link: https://lore.kernel.org/r/20220512215045.489140-1-stephen.s.brennan@oracle.com/ # v2
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
As noted in the "Deprecated Interfaces, Language Features, Attributes,
and Conventions" documentation [1], size calculations (especially
multiplication) should not be performed in memory allocator (or similar)
function arguments due to the risk of them overflowing. This could lead
to values wrapping around and a smaller allocation being made than the
caller was expecting. Using those allocations could lead to linear
overflows of heap memory and other misbehaviors.
So, use the struct_size() helper to do the arithmetic instead of the
argument "size + count * size" in the kmalloc() and kzalloc() functions.
Also, take the opportunity to refactor the memcpy() calls to use the
struct_size() and flex_array_size() helpers.
[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
Signed-off-by: Len Baker <len.baker@gmx.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
This reverts commit 6a9dc5fd61 ("lib: Revert use of fallthrough
pseudo-keyword in lib/")
Now that we can build arch/powerpc/boot/ free of -Wimplicit-fallthrough,
re-enable these fixes for lib/.
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/236
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
The following build error for powerpc64 was reported by Nathan Chancellor:
"$ scripts/config --file arch/powerpc/configs/powernv_defconfig -e KERNEL_XZ
$ make -skj"$(nproc)" ARCH=powerpc CROSS_COMPILE=powerpc64le-linux- distclean powernv_defconfig zImage
...
In file included from arch/powerpc/boot/../../../lib/decompress_unxz.c:234,
from arch/powerpc/boot/decompress.c:38:
arch/powerpc/boot/../../../lib/xz/xz_dec_stream.c: In function 'dec_main':
arch/powerpc/boot/../../../lib/xz/xz_dec_stream.c:586:4: error: 'fallthrough' undeclared (first use in this function)
586 | fallthrough;
| ^~~~~~~~~~~
This will end up affecting distribution configurations such as Debian
and OpenSUSE according to my testing. I am not sure what the solution
is, the PowerPC wrapper does not set -D__KERNEL__ so I am not sure
that compiler_attributes.h can be safely included."
In order to avoid these sort of problems, it seems that the best
solution is to use /* fall through */ comments instead of the
fallthrough pseudo-keyword macro in lib/, for now.
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Fixes: df561f6688 ("treewide: Use fallthrough pseudo-keyword")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-and-tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Based on 1 normalized pattern(s):
this program is free software you can redistribute it and or modify
it under the terms of the gnu general public licence as published by
the free software foundation either version 2 of the licence or at
your option any later version
extracted by the scancode license scanner the SPDX license identifier
GPL-2.0-or-later
has been chosen to replace the boilerplate/reference in 114 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Allison Randal <allison@lohutok.net>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190520170857.552531963@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
This patch fixes the following warning:
lib/assoc_array.c: In function `assoc_array_delete':
lib/assoc_array.c:1110:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
^~~
lib/assoc_array.c:1118:2: note: here
case assoc_array_walk_tree_empty:
^~~~
Warning level 3 was used: -Wimplicit-fallthrough=3
This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Link: http://lkml.kernel.org/r/20190212212206.GA16378@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Fix the creation of shortcuts for which the length of the index key value
is an exact multiple of the machine word size. The problem is that the
code that blanks off the unused bits of the shortcut value malfunctions if
the number of bits in the last word equals machine word size. This is due
to the "<<" operator being given a shift of zero in this case, and so the
mask that should be all zeros is all ones instead. This causes the
subsequent masking operation to clear everything rather than clearing
nothing.
Ordinarily, the presence of the hash at the beginning of the tree index key
makes the issue very hard to test for, but in this case, it was encountered
due to a development mistake that caused the hash output to be either 0
(keyring) or 1 (non-keyring) only. This made it susceptible to the
keyctl/unlink/valid test in the keyutils package.
The fix is simply to skip the blanking if the shift would be 0. For
example, an index key that is 64 bits long would produce a 0 shift and thus
a 'blank' of all 1s. This would then be inverted and AND'd onto the
index_key, incorrectly clearing the entire last word.
Fixes: 3cb989501c ("Add a generic associative array implementation.")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.morris@microsoft.com>
Now that smp_read_barrier_depends() is implied by READ_ONCE(), the several
smp_read_barrier_depends() calls may be removed from lib/assoc_array.c.
This commit makes this change and marks the READ_ONCE() calls that head
address dependencies.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Kuleshov <kuleshovmail@gmail.com>
Cc: David Howells <dhowells@redhat.com>
This fixes CVE-2017-12193.
Fix a case in the assoc_array implementation in which a new leaf is
added that needs to go into a node that happens to be full, where the
existing leaves in that node cluster together at that level to the
exclusion of new leaf.
What needs to happen is that the existing leaves get moved out to a new
node, N1, at level + 1 and the existing node needs replacing with one,
N0, that has pointers to the new leaf and to N1.
The code that tries to do this gets this wrong in two ways:
(1) The pointer that should've pointed from N0 to N1 is set to point
recursively to N0 instead.
(2) The backpointer from N0 needs to be set correctly in the case N0 is
either the root node or reached through a shortcut.
Fix this by removing this path and using the split_node path instead,
which achieves the same end, but in a more general way (thanks to Eric
Biggers for spotting the redundancy).
The problem manifests itself as:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: assoc_array_apply_edit+0x59/0xe5
Fixes: 3cb989501c ("Add a generic associative array implementation.")
Reported-and-tested-by: WU Fan <u3536072@connect.hku.hk>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: stable@vger.kernel.org [v3.13-rc1+]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Please do not apply this to mainline directly, instead please re-run the
coccinelle script shown below and apply its output.
For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
preference to ACCESS_ONCE(), and new code is expected to use one of the
former. So far, there's been no reason to change most existing uses of
ACCESS_ONCE(), as these aren't harmful, and changing them results in
churn.
However, for some features, the read/write distinction is critical to
correct operation. To distinguish these cases, separate read/write
accessors must be used. This patch migrates (most) remaining
ACCESS_ONCE() instances to {READ,WRITE}_ONCE(), using the following
coccinelle script:
----
// Convert trivial ACCESS_ONCE() uses to equivalent READ_ONCE() and
// WRITE_ONCE()
// $ make coccicheck COCCI=/home/mark/once.cocci SPFLAGS="--include-headers" MODE=patch
virtual patch
@ depends on patch @
expression E1, E2;
@@
- ACCESS_ONCE(E1) = E2
+ WRITE_ONCE(E1, E2)
@ depends on patch @
expression E;
@@
- ACCESS_ONCE(E)
+ READ_ONCE(E)
----
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: davem@davemloft.net
Cc: linux-arch@vger.kernel.org
Cc: mpe@ellerman.id.au
Cc: shuah@kernel.org
Cc: snitzer@redhat.com
Cc: thor.thayer@linux.intel.com
Cc: tj@kernel.org
Cc: viro@zeniv.linux.org.uk
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1508792849-3115-19-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Include rcupdate.h header to provide call_rcu() definition. This was implicitly
being provided by slab.h file which include srcu.h somewhere in its include
hierarchy which in-turn included rcupdate.h.
Lately, tinification effort added support to remove srcu entirely because of
which we are encountering build errors like
lib/assoc_array.c: In function 'assoc_array_apply_edit':
lib/assoc_array.c:1426:2: error: implicit declaration of function 'call_rcu' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
Fix these by including rcupdate.h explicitly.
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reported-by: Scott Wood <scottwood@freescale.com>
This fixes CVE-2014-3631.
It is possible for an associative array to end up with a shortcut node at the
root of the tree if there are more than fan-out leaves in the tree, but they
all crowd into the same slot in the lowest level (ie. they all have the same
first nibble of their index keys).
When assoc_array_gc() returns back up the tree after scanning some leaves, it
can fall off of the root and crash because it assumes that the back pointer
from a shortcut (after label ascend_old_tree) must point to a normal node -
which isn't true of a shortcut node at the root.
Should we find we're ascending rootwards over a shortcut, we should check to
see if the backpointer is zero - and if it is, we have completed the scan.
This particular bug cannot occur if the root node is not a shortcut - ie. if
you have fewer than 17 keys in a keyring or if you have at least two keys that
sit into separate slots (eg. a keyring and a non keyring).
This can be reproduced by:
ring=`keyctl newring bar @s`
for ((i=1; i<=18; i++)); do last_key=`keyctl newring foo$i $ring`; done
keyctl timeout $last_key 2
Doing this:
echo 3 >/proc/sys/kernel/keys/gc_delay
first will speed things up.
If we do fall off of the top of the tree, we get the following oops:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
IP: [<ffffffff8136cea7>] assoc_array_gc+0x2f7/0x540
PGD dae15067 PUD cfc24067 PMD 0
Oops: 0000 [#1] SMP
Modules linked in: xt_nat xt_mark nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_rpfilter ip6t_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_ni
CPU: 0 PID: 26011 Comm: kworker/0:1 Not tainted 3.14.9-200.fc20.x86_64 #1
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Workqueue: events key_garbage_collector
task: ffff8800918bd580 ti: ffff8800aac14000 task.ti: ffff8800aac14000
RIP: 0010:[<ffffffff8136cea7>] [<ffffffff8136cea7>] assoc_array_gc+0x2f7/0x540
RSP: 0018:ffff8800aac15d40 EFLAGS: 00010206
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8800aaecacc0
RDX: ffff8800daecf440 RSI: 0000000000000001 RDI: ffff8800aadc2bc0
RBP: ffff8800aac15da8 R08: 0000000000000001 R09: 0000000000000003
R10: ffffffff8136ccc7 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000070 R15: 0000000000000001
FS: 0000000000000000(0000) GS:ffff88011fc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000018 CR3: 00000000db10d000 CR4: 00000000000006f0
Stack:
ffff8800aac15d50 0000000000000011 ffff8800aac15db8 ffffffff812e2a70
ffff880091a00600 0000000000000000 ffff8800aadc2bc3 00000000cd42c987
ffff88003702df20 ffff88003702dfa0 0000000053b65c09 ffff8800aac15fd8
Call Trace:
[<ffffffff812e2a70>] ? keyring_detect_cycle_iterator+0x30/0x30
[<ffffffff812e3e75>] keyring_gc+0x75/0x80
[<ffffffff812e1424>] key_garbage_collector+0x154/0x3c0
[<ffffffff810a67b6>] process_one_work+0x176/0x430
[<ffffffff810a744b>] worker_thread+0x11b/0x3a0
[<ffffffff810a7330>] ? rescuer_thread+0x3b0/0x3b0
[<ffffffff810ae1a8>] kthread+0xd8/0xf0
[<ffffffff810ae0d0>] ? insert_kthread_work+0x40/0x40
[<ffffffff816ffb7c>] ret_from_fork+0x7c/0xb0
[<ffffffff810ae0d0>] ? insert_kthread_work+0x40/0x40
Code: 08 4c 8b 22 0f 84 bf 00 00 00 41 83 c7 01 49 83 e4 fc 41 83 ff 0f 4c 89 65 c0 0f 8f 5a fe ff ff 48 8b 45 c0 4d 63 cf 49 83 c1 02 <4e> 8b 34 c8 4d 85 f6 0f 84 be 00 00 00 41 f6 c6 01 0f 84 92
RIP [<ffffffff8136cea7>] assoc_array_gc+0x2f7/0x540
RSP <ffff8800aac15d40>
CR2: 0000000000000018
---[ end trace 1129028a088c0cbd ]---
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
An edit script should be considered inaccessible by a function once it has
called assoc_array_apply_edit() or assoc_array_cancel_edit().
However, assoc_array_gc() is accessing the edit script just after the
gc_complete: label.
Reported-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
cc: shemming@brocade.com
cc: paulmck@linux.vnet.ibm.com
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>
The associative array code creates unnecessary and potentially
problematic global variable 'status'. Remove it since never used.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
If sufficient keys (or keyrings) are added into a keyring such that a node in
the associative array's tree overflows (each node has a capacity N, currently
16) and such that all N+1 keys have the same index key segment for that level
of the tree (the level'th nibble of the index key), then assoc_array_insert()
calls ops->diff_objects() to indicate at which bit position the two index keys
vary.
However, __key_link_begin() passes a NULL object to assoc_array_insert() with
the intention of supplying the correct pointer later before we commit the
change. This means that keyring_diff_objects() is given a NULL pointer as one
of its arguments which it does not expect. This results in an oops like the
attached.
With the previous patch to fix the keyring hash function, this can be forced
much more easily by creating a keyring and only adding keyrings to it. Add any
other sort of key and a different insertion path is taken - all 16+1 objects
must want to cluster in the same node slot.
This can be tested by:
r=`keyctl newring sandbox @s`
for ((i=0; i<=16; i++)); do keyctl newring ring$i $r; done
This should work fine, but oopses when the 17th keyring is added.
Since ops->diff_objects() is always called with the first pointer pointing to
the object to be inserted (ie. the NULL pointer), we can fix the problem by
changing the to-be-inserted object pointer to point to the index key passed
into assoc_array_insert() instead.
Whilst we're at it, we also switch the arguments so that they are the same as
for ->compare_object().
BUG: unable to handle kernel NULL pointer dereference at 0000000000000088
IP: [<ffffffff81191ee4>] hash_key_type_and_desc+0x18/0xb0
...
RIP: 0010:[<ffffffff81191ee4>] hash_key_type_and_desc+0x18/0xb0
...
Call Trace:
[<ffffffff81191f9d>] keyring_diff_objects+0x21/0xd2
[<ffffffff811f09ef>] assoc_array_insert+0x3b6/0x908
[<ffffffff811929a7>] __key_link_begin+0x78/0xe5
[<ffffffff81191a2e>] key_create_or_update+0x17d/0x36a
[<ffffffff81192e0a>] SyS_add_key+0x123/0x183
[<ffffffff81400ddb>] tracesys+0xdd/0xe2
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Stephen Gallagher <sgallagh@redhat.com>
Expand the capacity of a keyring to be able to hold a lot more keys by using
the previously added associative array implementation. Currently the maximum
capacity is:
(PAGE_SIZE - sizeof(header)) / sizeof(struct key *)
which, on a 64-bit system, is a little more 500. However, since this is being
used for the NFS uid mapper, we need more than that. The new implementation
gives us effectively unlimited capacity.
With some alterations, the keyutils testsuite runs successfully to completion
after this patch is applied. The alterations are because (a) keyrings that
are simply added to no longer appear ordered and (b) some of the errors have
changed a bit.
Signed-off-by: David Howells <dhowells@redhat.com>
Add a generic associative array implementation that can be used as the
container for keyrings, thereby massively increasing the capacity available
whilst also speeding up searching in keyrings that contain a lot of keys.
This may also be useful in FS-Cache for tracking cookies.
Documentation is added into Documentation/associative_array.txt
Some of the properties of the implementation are:
(1) Objects are opaque pointers. The implementation does not care where they
point (if anywhere) or what they point to (if anything).
[!] NOTE: Pointers to objects _must_ be zero in the two least significant
bits.
(2) Objects do not need to contain linkage blocks for use by the array. This
permits an object to be located in multiple arrays simultaneously.
Rather, the array is made up of metadata blocks that point to objects.
(3) Objects are labelled as being one of two types (the type is a bool value).
This information is stored in the array, but has no consequence to the
array itself or its algorithms.
(4) Objects require index keys to locate them within the array.
(5) Index keys must be unique. Inserting an object with the same key as one
already in the array will replace the old object.
(6) Index keys can be of any length and can be of different lengths.
(7) Index keys should encode the length early on, before any variation due to
length is seen.
(8) Index keys can include a hash to scatter objects throughout the array.
(9) The array can iterated over. The objects will not necessarily come out in
key order.
(10) The array can be iterated whilst it is being modified, provided the RCU
readlock is being held by the iterator. Note, however, under these
circumstances, some objects may be seen more than once. If this is a
problem, the iterator should lock against modification. Objects will not
be missed, however, unless deleted.
(11) Objects in the array can be looked up by means of their index key.
(12) Objects can be looked up whilst the array is being modified, provided the
RCU readlock is being held by the thread doing the look up.
The implementation uses a tree of 16-pointer nodes internally that are indexed
on each level by nibbles from the index key. To improve memory efficiency,
shortcuts can be emplaced to skip over what would otherwise be a series of
single-occupancy nodes. Further, nodes pack leaf object pointers into spare
space in the node rather than making an extra branch until as such time an
object needs to be added to a full node.
Signed-off-by: David Howells <dhowells@redhat.com>
Stephen Brennan