Kernel Lock Torture Test Operation

CONFIG_LOCK_TORTURE_TEST

The CONFIG LOCK_TORTURE_TEST config option provides a kernel module that runs torture tests on core kernel locking primitives. The kernel module, ‘locktorture’, may be built after the fact on the running kernel to be tested, if desired. The tests periodically output status messages via printk(), which can be examined via the dmesg (perhaps grepping for “torture”). The test is started when the module is loaded, and stops when the module is unloaded. This program is based on how RCU is tortured, via rcutorture.

This torture test consists of creating a number of kernel threads which acquire the lock and hold it for specific amount of time, thus simulating different critical region behaviors. The amount of contention on the lock can be simulated by either enlarging this critical region hold time and/or creating more kthreads.

Module Parameters

This module has the following parameters:

Locktorture-specific

nwriters_stress

Number of kernel threads that will stress exclusive lock ownership (writers). The default value is twice the number of online CPUs.

nreaders_stress

Number of kernel threads that will stress shared lock ownership (readers). The default is the same amount of writer locks. If the user did not specify nwriters_stress, then both readers and writers be the amount of online CPUs.

torture_type

Type of lock to torture. By default, only spinlocks will be tortured. This module can torture the following locks, with string values as follows:

  • “lock_busted”:

    Simulates a buggy lock implementation.

  • “spin_lock”:

    spin_lock() and spin_unlock() pairs.

  • “spin_lock_irq”:

    spin_lock_irq() and spin_unlock_irq() pairs.

  • “rw_lock”:

    read/write lock() and unlock() rwlock pairs.

  • “rw_lock_irq”:

    read/write lock_irq() and unlock_irq() rwlock pairs.

  • “mutex_lock”:

    mutex_lock() and mutex_unlock() pairs.

  • “rtmutex_lock”:

    rtmutex_lock() and rtmutex_unlock() pairs. Kernel must have CONFIG_RT_MUTEX=y.

  • “rwsem_lock”:

    read/write down() and up() semaphore pairs.

Torture-framework (RCU + locking)

shutdown_secs

The number of seconds to run the test before terminating the test and powering off the system. The default is zero, which disables test termination and system shutdown. This capability is useful for automated testing.

onoff_interval

The number of seconds between each attempt to execute a randomly selected CPU-hotplug operation. Defaults to zero, which disables CPU hotplugging. In CONFIG_HOTPLUG_CPU=n kernels, locktorture will silently refuse to do any CPU-hotplug operations regardless of what value is specified for onoff_interval.

onoff_holdoff

The number of seconds to wait until starting CPU-hotplug operations. This would normally only be used when locktorture was built into the kernel and started automatically at boot time, in which case it is useful in order to avoid confusing boot-time code with CPUs coming and going. This parameter is only useful if CONFIG_HOTPLUG_CPU is enabled.

stat_interval

Number of seconds between statistics-related printk()s. By default, locktorture will report stats every 60 seconds. Setting the interval to zero causes the statistics to be printed -only- when the module is unloaded.

stutter

The length of time to run the test before pausing for this same period of time. Defaults to “stutter=5”, so as to run and pause for (roughly) five-second intervals. Specifying “stutter=0” causes the test to run continuously without pausing.

shuffle_interval

The number of seconds to keep the test threads affinitied to a particular subset of the CPUs, defaults to 3 seconds. Used in conjunction with test_no_idle_hz.

verbose

Enable verbose debugging printing, via printk(). Enabled by default. This extra information is mostly related to high-level errors and reports from the main ‘torture’ framework.

Statistics

Statistics are printed in the following format:

spin_lock-torture: Writes:  Total: 93746064  Max/Min: 0/0   Fail: 0
   (A)                    (B)            (C)            (D)          (E)

(A): Lock type that is being tortured -- torture_type parameter.

(B): Number of writer lock acquisitions. If dealing with a read/write
     primitive a second "Reads" statistics line is printed.

(C): Number of times the lock was acquired.

(D): Min and max number of times threads failed to acquire the lock.

(E): true/false values if there were errors acquiring the lock. This should
     -only- be positive if there is a bug in the locking primitive's
     implementation. Otherwise a lock should never fail (i.e., spin_lock()).
     Of course, the same applies for (C), above. A dummy example of this is
     the "lock_busted" type.

Usage

The following script may be used to torture locks:

#!/bin/sh

modprobe locktorture
sleep 3600
rmmod locktorture
dmesg | grep torture:

The output can be manually inspected for the error flag of “!!!”. One could of course create a more elaborate script that automatically checked for such errors. The “rmmod” command forces a “SUCCESS”, “FAILURE”, or “RCU_HOTPLUG” indication to be printk()ed. The first two are self-explanatory, while the last indicates that while there were no locking failures, CPU-hotplug problems were detected.

Also see: Documentation/RCU/torture.rst