bfcb2b538f
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
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