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Subject[git patch review 7/7] [IB] umad: further ib_unregister_mad_agent() deadlock fixes
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Date
The previous umad deadlock fix left ib_umad_kill_port() still
vulnerable to deadlocking. This patch fixes that by downgrading our
lock to a read lock when we might end up trying to reacquire the lock
for reading.

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

---
drivers/infiniband/core/user_mad.c | 87 ++++++++++++++++++++++++++----------
1 files changed, 63 insertions(+), 24 deletions(-)
applies-to: 17115437026be55dcd74641be21561fecf33dcdb
94382f3562e350ed7c8f7dcd6fc968bdece31328
diff --git a/drivers/infiniband/core/user_mad.c b/drivers/infiniband/core/user_mad.c
index d61f544..5ea741f 100644
--- a/drivers/infiniband/core/user_mad.c
+++ b/drivers/infiniband/core/user_mad.c
@@ -31,7 +31,7 @@
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*
- * $Id: user_mad.c 2814 2005-07-06 19:14:09Z halr $
+ * $Id: user_mad.c 4010 2005-11-09 23:11:56Z roland $
*/

#include <linux/module.h>
@@ -110,12 +110,13 @@ struct ib_umad_device {
};

struct ib_umad_file {
- struct ib_umad_port *port;
- struct list_head recv_list;
- struct list_head port_list;
- spinlock_t recv_lock;
- wait_queue_head_t recv_wait;
- struct ib_mad_agent *agent[IB_UMAD_MAX_AGENTS];
+ struct ib_umad_port *port;
+ struct list_head recv_list;
+ struct list_head port_list;
+ spinlock_t recv_lock;
+ wait_queue_head_t recv_wait;
+ struct ib_mad_agent *agent[IB_UMAD_MAX_AGENTS];
+ int agents_dead;
};

struct ib_umad_packet {
@@ -144,6 +145,12 @@ static void ib_umad_release_dev(struct k
kfree(dev);
}

+/* caller must hold port->mutex at least for reading */
+static struct ib_mad_agent *__get_agent(struct ib_umad_file *file, int id)
+{
+ return file->agents_dead ? NULL : file->agent[id];
+}
+
static int queue_packet(struct ib_umad_file *file,
struct ib_mad_agent *agent,
struct ib_umad_packet *packet)
@@ -151,10 +158,11 @@ static int queue_packet(struct ib_umad_f
int ret = 1;

down_read(&file->port->mutex);
+
for (packet->mad.hdr.id = 0;
packet->mad.hdr.id < IB_UMAD_MAX_AGENTS;
packet->mad.hdr.id++)
- if (agent == file->agent[packet->mad.hdr.id]) {
+ if (agent == __get_agent(file, packet->mad.hdr.id)) {
spin_lock_irq(&file->recv_lock);
list_add_tail(&packet->list, &file->recv_list);
spin_unlock_irq(&file->recv_lock);
@@ -326,7 +334,7 @@ static ssize_t ib_umad_write(struct file

down_read(&file->port->mutex);

- agent = file->agent[packet->mad.hdr.id];
+ agent = __get_agent(file, packet->mad.hdr.id);
if (!agent) {
ret = -EINVAL;
goto err_up;
@@ -480,7 +488,7 @@ static int ib_umad_reg_agent(struct ib_u
}

for (agent_id = 0; agent_id < IB_UMAD_MAX_AGENTS; ++agent_id)
- if (!file->agent[agent_id])
+ if (!__get_agent(file, agent_id))
goto found;

ret = -ENOMEM;
@@ -530,7 +538,7 @@ static int ib_umad_unreg_agent(struct ib

down_write(&file->port->mutex);

- if (id < 0 || id >= IB_UMAD_MAX_AGENTS || !file->agent[id]) {
+ if (id < 0 || id >= IB_UMAD_MAX_AGENTS || !__get_agent(file, id)) {
ret = -EINVAL;
goto out;
}
@@ -608,21 +616,29 @@ static int ib_umad_close(struct inode *i
struct ib_umad_file *file = filp->private_data;
struct ib_umad_device *dev = file->port->umad_dev;
struct ib_umad_packet *packet, *tmp;
+ int already_dead;
int i;

- for (i = 0; i < IB_UMAD_MAX_AGENTS; ++i)
- if (file->agent[i])
- ib_unregister_mad_agent(file->agent[i]);
+ down_write(&file->port->mutex);
+
+ already_dead = file->agents_dead;
+ file->agents_dead = 1;

list_for_each_entry_safe(packet, tmp, &file->recv_list, list)
kfree(packet);

- down_write(&file->port->mutex);
list_del(&file->port_list);
- up_write(&file->port->mutex);

- kfree(file);
+ downgrade_write(&file->port->mutex);
+
+ if (!already_dead)
+ for (i = 0; i < IB_UMAD_MAX_AGENTS; ++i)
+ if (file->agent[i])
+ ib_unregister_mad_agent(file->agent[i]);

+ up_read(&file->port->mutex);
+
+ kfree(file);
kref_put(&dev->ref, ib_umad_release_dev);

return 0;
@@ -848,13 +864,36 @@ static void ib_umad_kill_port(struct ib_

port->ib_dev = NULL;

- list_for_each_entry(file, &port->file_list, port_list)
- for (id = 0; id < IB_UMAD_MAX_AGENTS; ++id) {
- if (!file->agent[id])
- continue;
- ib_unregister_mad_agent(file->agent[id]);
- file->agent[id] = NULL;
- }
+ /*
+ * Now go through the list of files attached to this port and
+ * unregister all of their MAD agents. We need to hold
+ * port->mutex while doing this to avoid racing with
+ * ib_umad_close(), but we can't hold the mutex for writing
+ * while calling ib_unregister_mad_agent(), since that might
+ * deadlock by calling back into queue_packet(). So we
+ * downgrade our lock to a read lock, and then drop and
+ * reacquire the write lock for the next iteration.
+ *
+ * We do list_del_init() on the file's list_head so that the
+ * list_del in ib_umad_close() is still OK, even after the
+ * file is removed from the list.
+ */
+ while (!list_empty(&port->file_list)) {
+ file = list_entry(port->file_list.next, struct ib_umad_file,
+ port_list);
+
+ file->agents_dead = 1;
+ list_del_init(&file->port_list);
+
+ downgrade_write(&port->mutex);
+
+ for (id = 0; id < IB_UMAD_MAX_AGENTS; ++id)
+ if (file->agent[id])
+ ib_unregister_mad_agent(file->agent[id]);
+
+ up_read(&port->mutex);
+ down_write(&port->mutex);
+ }

up_write(&port->mutex);

---
0.99.9e
-
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