mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-06 02:24:14 +08:00
992d7ced75
From source code of get_usage_char(), the previous note is not correct, so fix it. static char get_usage_char(struct lock_class *class, enum lock_usage_bit bit) { char c = '.'; if (class->usage_mask & lock_flag(bit + 2))/*LOCK_ENABLED_##STATE*/ c = '+'; if (class->usage_mask & lock_flag(bit)) {/*LOCK_USED_IN_##STATE*/ c = '-'; if (class->usage_mask & lock_flag(bit + 2)) c = '?'; } return c; } note: 1) The 'bit' parameter always is passed as LOCK_USED_IN_##STATE or LOCK_USED_IN_##STATE_READ , from get_usage_chars(). Signed-off-by: Ming Lei <tom.leiming@gmail.com> LKML-Reference: <1240585806-5744-1-git-send-email-tom.leiming@gmail.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
224 lines
8.8 KiB
Plaintext
224 lines
8.8 KiB
Plaintext
Runtime locking correctness validator
|
|
=====================================
|
|
|
|
started by Ingo Molnar <mingo@redhat.com>
|
|
additions by Arjan van de Ven <arjan@linux.intel.com>
|
|
|
|
Lock-class
|
|
----------
|
|
|
|
The basic object the validator operates upon is a 'class' of locks.
|
|
|
|
A class of locks is a group of locks that are logically the same with
|
|
respect to locking rules, even if the locks may have multiple (possibly
|
|
tens of thousands of) instantiations. For example a lock in the inode
|
|
struct is one class, while each inode has its own instantiation of that
|
|
lock class.
|
|
|
|
The validator tracks the 'state' of lock-classes, and it tracks
|
|
dependencies between different lock-classes. The validator maintains a
|
|
rolling proof that the state and the dependencies are correct.
|
|
|
|
Unlike an lock instantiation, the lock-class itself never goes away: when
|
|
a lock-class is used for the first time after bootup it gets registered,
|
|
and all subsequent uses of that lock-class will be attached to this
|
|
lock-class.
|
|
|
|
State
|
|
-----
|
|
|
|
The validator tracks lock-class usage history into 4n + 1 separate state bits:
|
|
|
|
- 'ever held in STATE context'
|
|
- 'ever head as readlock in STATE context'
|
|
- 'ever head with STATE enabled'
|
|
- 'ever head as readlock with STATE enabled'
|
|
|
|
Where STATE can be either one of (kernel/lockdep_states.h)
|
|
- hardirq
|
|
- softirq
|
|
- reclaim_fs
|
|
|
|
- 'ever used' [ == !unused ]
|
|
|
|
When locking rules are violated, these state bits are presented in the
|
|
locking error messages, inside curlies. A contrived example:
|
|
|
|
modprobe/2287 is trying to acquire lock:
|
|
(&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
|
|
|
|
but task is already holding lock:
|
|
(&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
|
|
|
|
|
|
The bit position indicates STATE, STATE-read, for each of the states listed
|
|
above, and the character displayed in each indicates:
|
|
|
|
'.' acquired while irqs disabled and not in irq context
|
|
'-' acquired in irq context
|
|
'+' acquired with irqs enabled
|
|
'?' acquired in irq context with irqs enabled.
|
|
|
|
Unused mutexes cannot be part of the cause of an error.
|
|
|
|
|
|
Single-lock state rules:
|
|
------------------------
|
|
|
|
A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The
|
|
following states are exclusive, and only one of them is allowed to be
|
|
set for any lock-class:
|
|
|
|
<hardirq-safe> and <hardirq-unsafe>
|
|
<softirq-safe> and <softirq-unsafe>
|
|
|
|
The validator detects and reports lock usage that violate these
|
|
single-lock state rules.
|
|
|
|
Multi-lock dependency rules:
|
|
----------------------------
|
|
|
|
The same lock-class must not be acquired twice, because this could lead
|
|
to lock recursion deadlocks.
|
|
|
|
Furthermore, two locks may not be taken in different order:
|
|
|
|
<L1> -> <L2>
|
|
<L2> -> <L1>
|
|
|
|
because this could lead to lock inversion deadlocks. (The validator
|
|
finds such dependencies in arbitrary complexity, i.e. there can be any
|
|
other locking sequence between the acquire-lock operations, the
|
|
validator will still track all dependencies between locks.)
|
|
|
|
Furthermore, the following usage based lock dependencies are not allowed
|
|
between any two lock-classes:
|
|
|
|
<hardirq-safe> -> <hardirq-unsafe>
|
|
<softirq-safe> -> <softirq-unsafe>
|
|
|
|
The first rule comes from the fact the a hardirq-safe lock could be
|
|
taken by a hardirq context, interrupting a hardirq-unsafe lock - and
|
|
thus could result in a lock inversion deadlock. Likewise, a softirq-safe
|
|
lock could be taken by an softirq context, interrupting a softirq-unsafe
|
|
lock.
|
|
|
|
The above rules are enforced for any locking sequence that occurs in the
|
|
kernel: when acquiring a new lock, the validator checks whether there is
|
|
any rule violation between the new lock and any of the held locks.
|
|
|
|
When a lock-class changes its state, the following aspects of the above
|
|
dependency rules are enforced:
|
|
|
|
- if a new hardirq-safe lock is discovered, we check whether it
|
|
took any hardirq-unsafe lock in the past.
|
|
|
|
- if a new softirq-safe lock is discovered, we check whether it took
|
|
any softirq-unsafe lock in the past.
|
|
|
|
- if a new hardirq-unsafe lock is discovered, we check whether any
|
|
hardirq-safe lock took it in the past.
|
|
|
|
- if a new softirq-unsafe lock is discovered, we check whether any
|
|
softirq-safe lock took it in the past.
|
|
|
|
(Again, we do these checks too on the basis that an interrupt context
|
|
could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which
|
|
could lead to a lock inversion deadlock - even if that lock scenario did
|
|
not trigger in practice yet.)
|
|
|
|
Exception: Nested data dependencies leading to nested locking
|
|
-------------------------------------------------------------
|
|
|
|
There are a few cases where the Linux kernel acquires more than one
|
|
instance of the same lock-class. Such cases typically happen when there
|
|
is some sort of hierarchy within objects of the same type. In these
|
|
cases there is an inherent "natural" ordering between the two objects
|
|
(defined by the properties of the hierarchy), and the kernel grabs the
|
|
locks in this fixed order on each of the objects.
|
|
|
|
An example of such an object hierarchy that results in "nested locking"
|
|
is that of a "whole disk" block-dev object and a "partition" block-dev
|
|
object; the partition is "part of" the whole device and as long as one
|
|
always takes the whole disk lock as a higher lock than the partition
|
|
lock, the lock ordering is fully correct. The validator does not
|
|
automatically detect this natural ordering, as the locking rule behind
|
|
the ordering is not static.
|
|
|
|
In order to teach the validator about this correct usage model, new
|
|
versions of the various locking primitives were added that allow you to
|
|
specify a "nesting level". An example call, for the block device mutex,
|
|
looks like this:
|
|
|
|
enum bdev_bd_mutex_lock_class
|
|
{
|
|
BD_MUTEX_NORMAL,
|
|
BD_MUTEX_WHOLE,
|
|
BD_MUTEX_PARTITION
|
|
};
|
|
|
|
mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION);
|
|
|
|
In this case the locking is done on a bdev object that is known to be a
|
|
partition.
|
|
|
|
The validator treats a lock that is taken in such a nested fashion as a
|
|
separate (sub)class for the purposes of validation.
|
|
|
|
Note: When changing code to use the _nested() primitives, be careful and
|
|
check really thoroughly that the hierarchy is correctly mapped; otherwise
|
|
you can get false positives or false negatives.
|
|
|
|
Proof of 100% correctness:
|
|
--------------------------
|
|
|
|
The validator achieves perfect, mathematical 'closure' (proof of locking
|
|
correctness) in the sense that for every simple, standalone single-task
|
|
locking sequence that occurred at least once during the lifetime of the
|
|
kernel, the validator proves it with a 100% certainty that no
|
|
combination and timing of these locking sequences can cause any class of
|
|
lock related deadlock. [*]
|
|
|
|
I.e. complex multi-CPU and multi-task locking scenarios do not have to
|
|
occur in practice to prove a deadlock: only the simple 'component'
|
|
locking chains have to occur at least once (anytime, in any
|
|
task/context) for the validator to be able to prove correctness. (For
|
|
example, complex deadlocks that would normally need more than 3 CPUs and
|
|
a very unlikely constellation of tasks, irq-contexts and timings to
|
|
occur, can be detected on a plain, lightly loaded single-CPU system as
|
|
well!)
|
|
|
|
This radically decreases the complexity of locking related QA of the
|
|
kernel: what has to be done during QA is to trigger as many "simple"
|
|
single-task locking dependencies in the kernel as possible, at least
|
|
once, to prove locking correctness - instead of having to trigger every
|
|
possible combination of locking interaction between CPUs, combined with
|
|
every possible hardirq and softirq nesting scenario (which is impossible
|
|
to do in practice).
|
|
|
|
[*] assuming that the validator itself is 100% correct, and no other
|
|
part of the system corrupts the state of the validator in any way.
|
|
We also assume that all NMI/SMM paths [which could interrupt
|
|
even hardirq-disabled codepaths] are correct and do not interfere
|
|
with the validator. We also assume that the 64-bit 'chain hash'
|
|
value is unique for every lock-chain in the system. Also, lock
|
|
recursion must not be higher than 20.
|
|
|
|
Performance:
|
|
------------
|
|
|
|
The above rules require _massive_ amounts of runtime checking. If we did
|
|
that for every lock taken and for every irqs-enable event, it would
|
|
render the system practically unusably slow. The complexity of checking
|
|
is O(N^2), so even with just a few hundred lock-classes we'd have to do
|
|
tens of thousands of checks for every event.
|
|
|
|
This problem is solved by checking any given 'locking scenario' (unique
|
|
sequence of locks taken after each other) only once. A simple stack of
|
|
held locks is maintained, and a lightweight 64-bit hash value is
|
|
calculated, which hash is unique for every lock chain. The hash value,
|
|
when the chain is validated for the first time, is then put into a hash
|
|
table, which hash-table can be checked in a lockfree manner. If the
|
|
locking chain occurs again later on, the hash table tells us that we
|
|
dont have to validate the chain again.
|