From: Roland Dreier <rolandd@cisco.com>
Date: Fri, 11 Aug 2006 15:56:57 +0000 (-0700)
Subject: IB/mthca: Fix potential AB-BA deadlock with CQ locks
X-Git-Tag: v2.6.18-rc5~64^2~3
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a19aa5c5fdda8b556ab238177ee27c5ef7873c94;p=pandora-kernel.git

IB/mthca: Fix potential AB-BA deadlock with CQ locks

When destroying a QP, mthca locks both the QP's send CQ and receive
CQ.  However, the following scenario is perfectly valid:

    QP_a: send_cq == CQ_x, recv_cq == CQ_y
    QP_b: send_cq == CQ_y, recv_cq == CQ_x

The old mthca code simply locked send_cq and then recv_cq, which in
this case could lead to an AB-BA deadlock if QP_a and QP_b were
destroyed simultaneously.

We can fix this by changing the locking code to lock the CQ with the
lower CQ number first, which will create a consistent lock ordering.
Also, the second CQ is locked with spin_lock_nested() to tell lockdep
that we know what we're doing with the lock nesting.

This bug was found by lockdep.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
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