lockdep: Update documentation for lock-class leak detection

There are a number of bugs that can leak or overuse lock classes,
which can cause the maximum number of lock classes (currently 8191)
to be exceeded.  However, the documentation does not tell you how to
track down these problems.  This commit addresses this shortcoming.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2011-09-28 10:23:39 -07:00
parent 7077714ec4
commit b804cb9e91

View file

@ -221,3 +221,66 @@ 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.
Troubleshooting:
----------------
The validator tracks a maximum of MAX_LOCKDEP_KEYS number of lock classes.
Exceeding this number will trigger the following lockdep warning:
(DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
By default, MAX_LOCKDEP_KEYS is currently set to 8191, and typical
desktop systems have less than 1,000 lock classes, so this warning
normally results from lock-class leakage or failure to properly
initialize locks. These two problems are illustrated below:
1. Repeated module loading and unloading while running the validator
will result in lock-class leakage. The issue here is that each
load of the module will create a new set of lock classes for
that module's locks, but module unloading does not remove old
classes (see below discussion of reuse of lock classes for why).
Therefore, if that module is loaded and unloaded repeatedly,
the number of lock classes will eventually reach the maximum.
2. Using structures such as arrays that have large numbers of
locks that are not explicitly initialized. For example,
a hash table with 8192 buckets where each bucket has its own
spinlock_t will consume 8192 lock classes -unless- each spinlock
is explicitly initialized at runtime, for example, using the
run-time spin_lock_init() as opposed to compile-time initializers
such as __SPIN_LOCK_UNLOCKED(). Failure to properly initialize
the per-bucket spinlocks would guarantee lock-class overflow.
In contrast, a loop that called spin_lock_init() on each lock
would place all 8192 locks into a single lock class.
The moral of this story is that you should always explicitly
initialize your locks.
One might argue that the validator should be modified to allow
lock classes to be reused. However, if you are tempted to make this
argument, first review the code and think through the changes that would
be required, keeping in mind that the lock classes to be removed are
likely to be linked into the lock-dependency graph. This turns out to
be harder to do than to say.
Of course, if you do run out of lock classes, the next thing to do is
to find the offending lock classes. First, the following command gives
you the number of lock classes currently in use along with the maximum:
grep "lock-classes" /proc/lockdep_stats
This command produces the following output on a modest system:
lock-classes: 748 [max: 8191]
If the number allocated (748 above) increases continually over time,
then there is likely a leak. The following command can be used to
identify the leaking lock classes:
grep "BD" /proc/lockdep
Run the command and save the output, then compare against the output from
a later run of this command to identify the leakers. This same output
can also help you find situations where runtime lock initialization has
been omitted.