8f04da2adb
shrink_dentry_list() holds dentry->d_lock and needs to acquire
dentry->d_inode->i_lock. This cannot be done with a spin_lock()
operation because it's the reverse of the regular lock order.
To avoid ABBA deadlocks it is done with a trylock loop.
Trylock loops are problematic in two scenarios:
1) PREEMPT_RT converts spinlocks to 'sleeping' spinlocks, which are
preemptible. As a consequence the i_lock holder can be preempted
by a higher priority task. If that task executes the trylock loop
it will do so forever and live lock.
2) In virtual machines trylock loops are problematic as well. The
VCPU on which the i_lock holder runs can be scheduled out and a
task on a different VCPU can loop for a whole time slice. In the
worst case this can lead to starvation. Commits 47be61845c
("fs/dcache.c: avoid soft-lockup in dput()") and 046b961b45
("shrink_dentry_list(): take parent's d_lock earlier") are
addressing exactly those symptoms.
Avoid the trylock loop by using dentry_kill(). When pruning ancestors,
the same code applies that is used to kill a dentry in dput(). This
also has the benefit that the locking order is now the same. First
the inode is locked, then the parent.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. 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.