IB/qib: fix mcast detach when qp not attached

The code produces the following trace:

[1750924.419007] general protection fault: 0000 [#3] SMP
[1750924.420364] Modules linked in: nfnetlink autofs4 rpcsec_gss_krb5 nfsv4
dcdbas rfcomm bnep bluetooth nfsd auth_rpcgss nfs_acl dm_multipath nfs lockd
scsi_dh sunrpc fscache radeon ttm drm_kms_helper drm serio_raw parport_pc
ppdev i2c_algo_bit lpc_ich ipmi_si ib_mthca ib_qib dca lp parport ib_ipoib
mac_hid ib_cm i3000_edac ib_sa ib_uverbs edac_core ib_umad ib_mad ib_core
ib_addr tg3 ptp dm_mirror dm_region_hash dm_log psmouse pps_core
[1750924.420364] CPU: 1 PID: 8401 Comm: python Tainted: G D
3.13.0-39-generic #66-Ubuntu
[1750924.420364] Hardware name: Dell Computer Corporation PowerEdge
860/0XM089, BIOS A04 07/24/2007
[1750924.420364] task: ffff8800366a9800 ti: ffff88007af1c000 task.ti:
ffff88007af1c000
[1750924.420364] RIP: 0010:[<ffffffffa0131d51>] [<ffffffffa0131d51>]
qib_mcast_qp_free+0x11/0x50 [ib_qib]
[1750924.420364] RSP: 0018:ffff88007af1dd70  EFLAGS: 00010246
[1750924.420364] RAX: 0000000000000001 RBX: ffff88007b822688 RCX:
000000000000000f
[1750924.420364] RDX: ffff88007b822688 RSI: ffff8800366c15a0 RDI:
6764697200000000
[1750924.420364] RBP: ffff88007af1dd78 R08: 0000000000000001 R09:
0000000000000000
[1750924.420364] R10: 0000000000000011 R11: 0000000000000246 R12:
ffff88007baa1d98
[1750924.420364] R13: ffff88003ecab000 R14: ffff88007b822660 R15:
0000000000000000
[1750924.420364] FS:  00007ffff7fd8740(0000) GS:ffff88007fc80000(0000)
knlGS:0000000000000000
[1750924.420364] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[1750924.420364] CR2: 00007ffff597c750 CR3: 000000006860b000 CR4:
00000000000007e0
[1750924.420364] Stack:
[1750924.420364]  ffff88007b822688 ffff88007af1ddf0 ffffffffa0132429
000000007af1de20
[1750924.420364]  ffff88007baa1dc8 ffff88007baa0000 ffff88007af1de70
ffffffffa00cb313
[1750924.420364]  00007fffffffde88 0000000000000000 0000000000000008
ffff88003ecab000
[1750924.420364] Call Trace:
[1750924.420364]  [<ffffffffa0132429>] qib_multicast_detach+0x1e9/0x350
[ib_qib]
[1750924.568035]  [<ffffffffa00cb313>] ? ib_uverbs_modify_qp+0x323/0x3d0
[ib_uverbs]
[1750924.568035]  [<ffffffffa0092d61>] ib_detach_mcast+0x31/0x50 [ib_core]
[1750924.568035]  [<ffffffffa00cc213>] ib_uverbs_detach_mcast+0x93/0x170
[ib_uverbs]
[1750924.568035]  [<ffffffffa00c61f6>] ib_uverbs_write+0xc6/0x2c0 [ib_uverbs]
[1750924.568035]  [<ffffffff81312e68>] ? apparmor_file_permission+0x18/0x20
[1750924.568035]  [<ffffffff812d4cd3>] ? security_file_permission+0x23/0xa0
[1750924.568035]  [<ffffffff811bd214>] vfs_write+0xb4/0x1f0
[1750924.568035]  [<ffffffff811bdc49>] SyS_write+0x49/0xa0
[1750924.568035]  [<ffffffff8172f7ed>] system_call_fastpath+0x1a/0x1f
[1750924.568035] Code: 66 2e 0f 1f 84 00 00 00 00 00 31 c0 5d c3 66 2e 0f 1f
84 00 00 00 00 00 66 90 0f 1f 44 00 00 55 48 89 e5 53 48 89 fb 48 8b 7f 10
<f0> ff 8f 40 01 00 00 74 0e 48 89 df e8 8e f8 06 e1 5b 5d c3 0f
[1750924.568035] RIP  [<ffffffffa0131d51>] qib_mcast_qp_free+0x11/0x50
[ib_qib]
[1750924.568035]  RSP <ffff88007af1dd70>
[1750924.650439] ---[ end trace 73d5d4b3f8ad4851 ]

The fix is to note the qib_mcast_qp that was found.   If none is found, then
return EINVAL indicating the error.

Cc: <stable@vger.kernel.org>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reported-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/qib/qib_verbs_mcast.c

@@ -286,15 +286,13 @@ int qib_multicast_detach(struct ib_qp *ibqp, union ib_gid *gid, u16 lid)
 	struct qib_ibdev *dev = to_idev(ibqp->device);
 	struct qib_ibport *ibp = to_iport(ibqp->device, qp->port_num);
 	struct qib_mcast *mcast = NULL;
-	struct qib_mcast_qp *p, *tmp;
+	struct qib_mcast_qp *p, *tmp, *delp = NULL;
 	struct rb_node *n;
 	int last = 0;
 	int ret;
 
-	if (ibqp->qp_num <= 1 || qp->state == IB_QPS_RESET) {
-		ret = -EINVAL;
-		goto bail;
-	}
+	if (ibqp->qp_num <= 1 || qp->state == IB_QPS_RESET)
+		return -EINVAL;
 
 	spin_lock_irq(&ibp->lock);
 
@@ -303,8 +301,7 @@ int qib_multicast_detach(struct ib_qp *ibqp, union ib_gid *gid, u16 lid)
 	while (1) {
 		if (n == NULL) {
 			spin_unlock_irq(&ibp->lock);
-			ret = -EINVAL;
-			goto bail;
+			return -EINVAL;
 		}
 
 		mcast = rb_entry(n, struct qib_mcast, rb_node);
@@ -328,6 +325,7 @@ int qib_multicast_detach(struct ib_qp *ibqp, union ib_gid *gid, u16 lid)
 		 */
 		list_del_rcu(&p->list);
 		mcast->n_attached--;
+		delp = p;
 
 		/* If this was the last attached QP, remove the GID too. */
 		if (list_empty(&mcast->qp_list)) {
@@ -338,15 +336,16 @@ int qib_multicast_detach(struct ib_qp *ibqp, union ib_gid *gid, u16 lid)
 	}
 
 	spin_unlock_irq(&ibp->lock);
-
-	if (p) {
+	/* QP not attached */
+	if (!delp)
+		return -EINVAL;
 	/*
 	 * Wait for any list walkers to finish before freeing the
 	 * list element.
 	 */
 	wait_event(mcast->wait, atomic_read(&mcast->refcount) <= 1);
-		qib_mcast_qp_free(p);
-	}
+	qib_mcast_qp_free(delp);
+
 	if (last) {
 		atomic_dec(&mcast->refcount);
 		wait_event(mcast->wait, !atomic_read(&mcast->refcount));
@@ -355,11 +354,7 @@ int qib_multicast_detach(struct ib_qp *ibqp, union ib_gid *gid, u16 lid)
 		dev->n_mcast_grps_allocated--;
 		spin_unlock_irq(&dev->n_mcast_grps_lock);
 	}
-
-	ret = 0;
-
-bail:
-	return ret;
+	return 0;
 }
 
 int qib_mcast_tree_empty(struct qib_ibport *ibp)