From e214a0fe2b382fa302c036ecd6e6ffe99e3b9875 Mon Sep 17 00:00:00 2001
From: Eli Cohen <eli@dev.mellanox.co.il>
Date: Tue, 3 Jan 2012 20:36:48 -0800
Subject: [PATCH] IB/uverbs: Protect QP multicast list

Userspace verbs multicast attach/detach operations on a QP are done
while holding the rwsem of the QP for reading.  That's not sufficient
since a reader lock allows more than one reader to acquire the
lock.  However, multicast attach/detach does list manipulation that
can corrupt the list if multiple threads run in parallel.

Fix this by acquiring the rwsem as a writer to serialize attach/detach
operations.  Add idr_write_qp() and put_qp_write() to encapsulate
this.

This fixes oops seen when running applications that perform multicast
joins/leaves.

Reported by: Mike Dubman <miked@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>
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