Since XRC support was added, the uverbs code has locked SRQ, CQ and PD
objects needed during QP and SRQ creation in different orders
depending on the the code path.  This leads to the (at least
theoretical) possibility of deadlock, and triggers the lockdep splat
below.

Fix this by making sure we always lock the SRQ first, then CQs and
finally the PD.

    ======================================================
    [ INFO: possible circular locking dependency detected ]
    3.4.0-rc5+ #34 Not tainted
    -------------------------------------------------------
    ibv_srq_pingpon/2484 is trying to acquire lock:
     (SRQ-uobj){+++++.}, at: [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]

    but task is already holding lock:
     (CQ-uobj){+++++.}, at: [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]

    which lock already depends on the new lock.

    the existing dependency chain (in reverse order) is:

    -> #2 (CQ-uobj){+++++.}:
           [<ffffffff81070fd0>] lock_acquire+0xbf/0xfe
           [<ffffffff81384f28>] down_read+0x34/0x43
           [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]
           [<ffffffffa00af542>] idr_read_obj+0x9/0x19 [ib_uverbs]
           [<ffffffffa00b16c3>] ib_uverbs_create_qp+0x180/0x684 [ib_uverbs]
           [<ffffffffa00ae3dd>] ib_uverbs_write+0xb7/0xc2 [ib_uverbs]
           [<ffffffff810fe47f>] vfs_write+0xa7/0xee
           [<ffffffff810fe65f>] sys_write+0x45/0x69
           [<ffffffff8138cdf9>] system_call_fastpath+0x16/0x1b

    -> #1 (PD-uobj){++++++}:
           [<ffffffff81070fd0>] lock_acquire+0xbf/0xfe
           [<ffffffff81384f28>] down_read+0x34/0x43
           [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]
           [<ffffffffa00af542>] idr_read_obj+0x9/0x19 [ib_uverbs]
           [<ffffffffa00af8ad>] __uverbs_create_xsrq+0x96/0x386 [ib_uverbs]
           [<ffffffffa00b31b9>] ib_uverbs_detach_mcast+0x1cd/0x1e6 [ib_uverbs]
           [<ffffffffa00ae3dd>] ib_uverbs_write+0xb7/0xc2 [ib_uverbs]
           [<ffffffff810fe47f>] vfs_write+0xa7/0xee
           [<ffffffff810fe65f>] sys_write+0x45/0x69
           [<ffffffff8138cdf9>] system_call_fastpath+0x16/0x1b

    -> #0 (SRQ-uobj){+++++.}:
           [<ffffffff81070898>] __lock_acquire+0xa29/0xd06
           [<ffffffff81070fd0>] lock_acquire+0xbf/0xfe
           [<ffffffff81384f28>] down_read+0x34/0x43
           [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]
           [<ffffffffa00af542>] idr_read_obj+0x9/0x19 [ib_uverbs]
           [<ffffffffa00b1728>] ib_uverbs_create_qp+0x1e5/0x684 [ib_uverbs]
           [<ffffffffa00ae3dd>] ib_uverbs_write+0xb7/0xc2 [ib_uverbs]
           [<ffffffff810fe47f>] vfs_write+0xa7/0xee
           [<ffffffff810fe65f>] sys_write+0x45/0x69
           [<ffffffff8138cdf9>] system_call_fastpath+0x16/0x1b

    other info that might help us debug this:

    Chain exists of:
      SRQ-uobj --> PD-uobj --> CQ-uobj

     Possible unsafe locking scenario:

           CPU0                    CPU1
           ----                    ----
      lock(CQ-uobj);
                                   lock(PD-uobj);
                                   lock(CQ-uobj);
      lock(SRQ-uobj);

     *** DEADLOCK ***

    3 locks held by ibv_srq_pingpon/2484:
     #0:  (QP-uobj){+.+...}, at: [<ffffffffa00b162c>] ib_uverbs_create_qp+0xe9/0x684 [ib_uverbs]
     #1:  (PD-uobj){++++++}, at: [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]
     #2:  (CQ-uobj){+++++.}, at: [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]

    stack backtrace:
    Pid: 2484, comm: ibv_srq_pingpon Not tainted 3.4.0-rc5+ #34
    Call Trace:
     [<ffffffff8137eff0>] print_circular_bug+0x1f8/0x209
     [<ffffffff81070898>] __lock_acquire+0xa29/0xd06
     [<ffffffffa00af37c>] ? __idr_get_uobj+0x20/0x5e [ib_uverbs]
     [<ffffffffa00af51b>] ? idr_read_uobj+0x2f/0x4d [ib_uverbs]
     [<ffffffff81070fd0>] lock_acquire+0xbf/0xfe
     [<ffffffffa00af51b>] ? idr_read_uobj+0x2f/0x4d [ib_uverbs]
     [<ffffffff81070eee>] ? lock_release+0x166/0x189
     [<ffffffff81384f28>] down_read+0x34/0x43
     [<ffffffffa00af51b>] ? idr_read_uobj+0x2f/0x4d [ib_uverbs]
     [<ffffffffa00af51b>] idr_read_uobj+0x2f/0x4d [ib_uverbs]
     [<ffffffffa00af542>] idr_read_obj+0x9/0x19 [ib_uverbs]
     [<ffffffffa00b1728>] ib_uverbs_create_qp+0x1e5/0x684 [ib_uverbs]
     [<ffffffff81070fec>] ? lock_acquire+0xdb/0xfe
     [<ffffffff81070c09>] ? lock_release_non_nested+0x94/0x213
     [<ffffffff810d470f>] ? might_fault+0x40/0x90
     [<ffffffff810d470f>] ? might_fault+0x40/0x90
     [<ffffffffa00ae3dd>] ib_uverbs_write+0xb7/0xc2 [ib_uverbs]
     [<ffffffff810fe47f>] vfs_write+0xa7/0xee
     [<ffffffff810ff736>] ? fget_light+0x3b/0x99
     [<ffffffff810fe65f>] sys_write+0x45/0x69
     [<ffffffff8138cdf9>] system_call_fastpath+0x16/0x1b
Reported-by: NOr Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>