fs/dcache: Avoid a try_lock loop in shrink_dentry_list()
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 47be6184 ("fs/dcache.c: avoid soft-lockup in dput()") and 046b961b ("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>
parent
f657a666
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