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With lockdep turned on, the following circular lock dependency problem was reported: [ 57.470040] ====================================================== [ 57.502900] WARNING: possible circular locking dependency detected [ 57.535208] 4.18.0-0.rc3.1.el8+7.x86_64+debug #1 Tainted: G [ 57.577761] ------------------------------------------------------ [ 57.609714] tuned/1505 is trying to acquire lock: [ 57.633808] 00000000559deec5 (cpu_hotplug_lock.rw_sem){++++}, at: store+0x27/0x120 [ 57.672880] [ 57.672880] but task is already holding lock: [ 57.702184] 000000002136ca64 (kn->count#118){++++}, at: kernfs_fop_write+0x1d0/0x410 [ 57.742176] [ 57.742176] which lock already depends on the new lock. [ 57.742176] [ 57.785220] [ 57.785220] the existing dependency chain (in reverse order) is: : [ 58.932512] other info that might help us debug this: [ 58.932512] [ 58.973344] Chain exists of: [ 58.973344] cpu_hotplug_lock.rw_sem --> subsys mutex#5 --> kn->count#118 [ 58.973344] [ 59.030795] Possible unsafe locking scenario: [ 59.030795] [ 59.061248] CPU0 CPU1 [ 59.085377] ---- ---- [ 59.108160] lock(kn->count#118); [ 59.124935] lock(subsys mutex#5); [ 59.156330] lock(kn->count#118); [ 59.186088] lock(cpu_hotplug_lock.rw_sem); [ 59.208541] [ 59.208541] *** DEADLOCK *** In the cpufreq_register_driver() function, the lock sequence is: cpus_read_lock --> kn->count For the cpufreq sysfs store method, the lock sequence is: kn->count --> cpus_read_lock These sequences are actually safe as they are taking a share lock on cpu_hotplug_lock. However, the current lockdep code doesn't check for share locking when detecting circular lock dependency. Fixing that could be a substantial effort. Instead, we can work around this problem by using cpus_read_trylock() in the store method which is much simpler. The chance of not getting the read lock is very small. If that happens, the userspace application that writes the sysfs file will get an error. Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.