From 7145c241a1bf2841952c3e297c4080b357b3e52d Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@redhat.com>
Date: Fri, 13 Feb 2015 08:24:41 -0500
Subject: [PATCH] dm crypt: avoid deadlock in mempools

Fix a theoretical deadlock introduced in the previous commit ("dm crypt:
don't allocate pages for a partial request").

The function crypt_alloc_buffer may be called concurrently.  If we allocate
from the mempool concurrently, there is a possibility of deadlock.  For
example, if we have mempool of 256 pages, two processes, each wanting
256, pages allocate from the mempool concurrently, it may deadlock in a
situation where both processes have allocated 128 pages and the mempool
is exhausted.

To avoid such a scenario we allocate the pages under a mutex.  In order
to not degrade performance with excessive locking, we try non-blocking
allocations without a mutex first and if that fails, we fallback to a
blocking allocations with a mutex.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
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