From ea4ebde02e08558b020c4b61bb9a4c0fcf63028e Mon Sep 17 00:00:00 2001
From: Chris Mason <clm@fb.com>
Date: Thu, 19 Jun 2014 14:16:52 -0700
Subject: [PATCH] Btrfs: fix deadlocks with trylock on tree nodes

The Btrfs tree trylock function is poorly named.  It always takes
the spinlock and backs off if the blocking lock is held.  This
can lead to surprising lockups because people expect it to really be a
trylock.

This commit makes it a pure trylock, both for the spinlock and the
blocking lock.  It also reworks the nested lock handling slightly to
avoid taking the read lock while a spinning write lock might be held.

Signed-off-by: Chris Mason <clm@fb.com>
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