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Add non-blocking trylock variants for the rangelock functions.

Description

Add non-blocking trylock variants for the rangelock functions.

A future patch that will add a Linux compatible copy_file_range(2) syscall
needs to be able to lock the byte ranges of two files concurrently.
To do this without a risk of deadlock, a non-blocking variant of
vn_rangelock_rlock() called vn_rangelock_tryrlock() was needed.
This patch adds this, along with vn_rangelock_trywlock(), in order to
do this.
The patch also adds a couple of comments, that I hope clarify how the
algorithm used in kern_rangelock.c works.

Reviewed by: kib, asomers (previous version)
Differential Revision: https://reviews.freebsd.org/D20645

Details

Committed
rmacklemJun 27 2019, 11:10 PM
Reviewer
kib
Differential Revision
D20645: add non-blocking variants of rangelock_rlock() and rangelock_wlock()
Parents
rS349475: Fix comment in sofree() to reference sbdestroy().
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