* [PATCH 02/25] IB/mthca, net/mlx4: remove counting semaphores
[not found] <1446000373-1823620-1-git-send-email-arnd@arndb.de>
@ 2015-10-28 2:45 ` Arnd Bergmann
[not found] ` <1446000373-1823620-3-git-send-email-arnd-r2nGTMty4D4@public.gmane.org>
2015-10-29 7:41 ` Jack Morgenstein
0 siblings, 2 replies; 3+ messages in thread
From: Arnd Bergmann @ 2015-10-28 2:45 UTC (permalink / raw)
To: tglx
Cc: Arnd Bergmann, Roland Dreier, Eli Cohen, Yevgeny Petrilin, netdev,
linux-rdma
The mthca and mlx4 device drivers use the same method
to switch between polling and event-driven command mode,
abusing two semaphores to create a mutual exclusion between
one polled command or multiple concurrent event driven
commands.
Since we want to make counting semaphores go away, this
patch replaces the semaphore counting the event-driven
commands with an open-coded wait-queue, which should
be an equivalent transformation of the code, although
it does not make it any nicer.
As far as I can tell, there is a preexisting race condition
regarding the cmd->use_events flag, which is not protected
by any lock. When this flag is toggled while another command
is being started, that command gets stuck until the mode is
toggled back.
A better solution that would solve the race condition and
at the same time improve the code readability would create
a new locking primitive that replaces both semaphores, like
static int mlx4_use_events(struct mlx4_cmd *cmd)
{
int ret = -EAGAIN;
spin_lock(&cmd->lock);
if (cmd->use_events && cmd->commands < cmd->max_commands) {
cmd->commands++;
ret = 1;
} else if (!cmd->use_events && cmd->commands == 0) {
cmd->commands = 1;
ret = 0;
}
spin_unlock(&cmd->lock);
return ret;
}
static bool mlx4_use_events(struct mlx4_cmd *cmd)
{
int ret;
wait_event(cmd->events_wq, ret = __mlx4_use_events(cmd) >= 0);
return ret;
}
Cc: Roland Dreier <roland@purestorage.com>
Cc: Eli Cohen <eli@mellanox.co.il>
Cc: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Cc: netdev@vger.kernel.org
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Conflicts:
drivers/net/mlx4/cmd.c
drivers/net/mlx4/mlx4.h
---
drivers/infiniband/hw/mthca/mthca_cmd.c | 12 ++++++++----
drivers/infiniband/hw/mthca/mthca_dev.h | 3 ++-
drivers/net/ethernet/mellanox/mlx4/cmd.c | 12 ++++++++----
drivers/net/ethernet/mellanox/mlx4/mlx4.h | 3 ++-
4 files changed, 20 insertions(+), 10 deletions(-)
diff --git a/drivers/infiniband/hw/mthca/mthca_cmd.c b/drivers/infiniband/hw/mthca/mthca_cmd.c
index 9d3e5c1ac60e..aad1852e8e10 100644
--- a/drivers/infiniband/hw/mthca/mthca_cmd.c
+++ b/drivers/infiniband/hw/mthca/mthca_cmd.c
@@ -417,7 +417,8 @@ static int mthca_cmd_wait(struct mthca_dev *dev,
int err = 0;
struct mthca_cmd_context *context;
- down(&dev->cmd.event_sem);
+ wait_event(dev->cmd.event_wait,
+ atomic_add_unless(&dev->cmd.commands, -1, 0));
spin_lock(&dev->cmd.context_lock);
BUG_ON(dev->cmd.free_head < 0);
@@ -459,7 +460,8 @@ out:
dev->cmd.free_head = context - dev->cmd.context;
spin_unlock(&dev->cmd.context_lock);
- up(&dev->cmd.event_sem);
+ atomic_inc(&dev->cmd.commands);
+ wake_up(&dev->cmd.event_wait);
return err;
}
@@ -571,7 +573,8 @@ int mthca_cmd_use_events(struct mthca_dev *dev)
dev->cmd.context[dev->cmd.max_cmds - 1].next = -1;
dev->cmd.free_head = 0;
- sema_init(&dev->cmd.event_sem, dev->cmd.max_cmds);
+ init_waitqueue_head(&dev->cmd.event_wait);
+ atomic_set(&dev->cmd.commands, dev->cmd.max_cmds);
spin_lock_init(&dev->cmd.context_lock);
for (dev->cmd.token_mask = 1;
@@ -597,7 +600,8 @@ void mthca_cmd_use_polling(struct mthca_dev *dev)
dev->cmd.flags &= ~MTHCA_CMD_USE_EVENTS;
for (i = 0; i < dev->cmd.max_cmds; ++i)
- down(&dev->cmd.event_sem);
+ wait_event(dev->cmd.event_wait,
+ atomic_add_unless(&dev->cmd.commands, -1, 0));
kfree(dev->cmd.context);
diff --git a/drivers/infiniband/hw/mthca/mthca_dev.h b/drivers/infiniband/hw/mthca/mthca_dev.h
index 7e6a6d64ad4e..3055f5c12ac8 100644
--- a/drivers/infiniband/hw/mthca/mthca_dev.h
+++ b/drivers/infiniband/hw/mthca/mthca_dev.h
@@ -121,7 +121,8 @@ struct mthca_cmd {
struct pci_pool *pool;
struct mutex hcr_mutex;
struct semaphore poll_sem;
- struct semaphore event_sem;
+ wait_queue_head_t event_wait;
+ atomic_t commands;
int max_cmds;
spinlock_t context_lock;
int free_head;
diff --git a/drivers/net/ethernet/mellanox/mlx4/cmd.c b/drivers/net/ethernet/mellanox/mlx4/cmd.c
index 78f5a1a0b8c8..60134a4245ef 100644
--- a/drivers/net/ethernet/mellanox/mlx4/cmd.c
+++ b/drivers/net/ethernet/mellanox/mlx4/cmd.c
@@ -273,7 +273,8 @@ static int mlx4_cmd_wait(struct mlx4_dev *dev, u64 in_param, u64 *out_param,
struct mlx4_cmd_context *context;
int err = 0;
- down(&cmd->event_sem);
+ wait_event(cmd->event_wait,
+ atomic_add_unless(&cmd->commands, -1, 0));
spin_lock(&cmd->context_lock);
BUG_ON(cmd->free_head < 0);
@@ -305,7 +306,8 @@ out:
cmd->free_head = context - cmd->context;
spin_unlock(&cmd->context_lock);
- up(&cmd->event_sem);
+ atomic_inc(&cmd->commands);
+ wake_up(&cmd->event_wait);
return err;
}
@@ -380,7 +382,8 @@ int mlx4_cmd_use_events(struct mlx4_dev *dev)
priv->cmd.context[priv->cmd.max_cmds - 1].next = -1;
priv->cmd.free_head = 0;
- sema_init(&priv->cmd.event_sem, priv->cmd.max_cmds);
+ init_waitqueue_head(&priv->cmd.event_wait);
+ atomic_set(&priv->cmd.commands, priv->cmd.max_cmds);
spin_lock_init(&priv->cmd.context_lock);
for (priv->cmd.token_mask = 1;
@@ -407,7 +410,8 @@ void mlx4_cmd_use_polling(struct mlx4_dev *dev)
priv->cmd.use_events = 0;
for (i = 0; i < priv->cmd.max_cmds; ++i)
- down(&priv->cmd.event_sem);
+ wait_event(priv->cmd.event_wait,
+ atomic_add_unless(&priv->cmd.commands, -1, 0));
kfree(priv->cmd.context);
diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4.h b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
index 5dfa68ffc11c..5a2c55d8ccfd 100644
--- a/drivers/net/ethernet/mellanox/mlx4/mlx4.h
+++ b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
@@ -189,7 +189,8 @@ struct mlx4_cmd {
void __iomem *hcr;
struct mutex hcr_mutex;
struct semaphore poll_sem;
- struct semaphore event_sem;
+ wait_queue_head_t event_wait;
+ atomic_t commands;
int max_cmds;
spinlock_t context_lock;
int free_head;
--
2.1.0.rc2
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH 02/25] IB/mthca, net/mlx4: remove counting semaphores
[not found] ` <1446000373-1823620-3-git-send-email-arnd-r2nGTMty4D4@public.gmane.org>
@ 2015-10-28 9:19 ` Sagi Grimberg
0 siblings, 0 replies; 3+ messages in thread
From: Sagi Grimberg @ 2015-10-28 9:19 UTC (permalink / raw)
To: Arnd Bergmann, tglx-hfZtesqFncYOwBW4kG4KsQ
Cc: Roland Dreier, Eli Cohen, Yevgeny Petrilin,
netdev-u79uwXL29TY76Z2rM5mHXA, linux-rdma-u79uwXL29TY76Z2rM5mHXA,
Nicholas A. Bellinger
Hi Arnd,
> Since we want to make counting semaphores go away,
Why do we want to make counting semaphores go away? completely?
or just for binary use cases?
I have a use case in iser target code where a counting semaphore is the
best suited synchronizing mechanism.
I have a single thread handling connect requests (one at a time) while
connect requests are event driven and come asynchronously. This is
why I use a queue and a counting semaphore to handle this situation.
I'd need to rethink of a new strategy to handle this without counting
semaphores and I'm not entirely sure it would be simpler.
> this patch replaces the semaphore counting the event-driven
> commands with an open-coded wait-queue, which should
> be an equivalent transformation of the code, although
> it does not make it any nicer.
>
> As far as I can tell, there is a preexisting race condition
> regarding the cmd->use_events flag, which is not protected
> by any lock. When this flag is toggled while another command
> is being started, that command gets stuck until the mode is
> toggled back.
>
> A better solution that would solve the race condition and
> at the same time improve the code readability would create
> a new locking primitive that replaces both semaphores, like
>
> static int mlx4_use_events(struct mlx4_cmd *cmd)
> {
> int ret = -EAGAIN;
> spin_lock(&cmd->lock);
> if (cmd->use_events && cmd->commands < cmd->max_commands) {
> cmd->commands++;
> ret = 1;
> } else if (!cmd->use_events && cmd->commands == 0) {
> cmd->commands = 1;
> ret = 0;
> }
> spin_unlock(&cmd->lock);
> return ret;
> }
>
> static bool mlx4_use_events(struct mlx4_cmd *cmd)
> {
> int ret;
> wait_event(cmd->events_wq, ret = __mlx4_use_events(cmd) >= 0);
> return ret;
> }
>
> Cc: Roland Dreier <roland-BHEL68pLQRGGvPXPguhicg@public.gmane.org>
> Cc: Eli Cohen <eli-VPRAkNaXOzVS1MOuV/RT9w@public.gmane.org>
> Cc: Yevgeny Petrilin <yevgenyp-VPRAkNaXOzVS1MOuV/RT9w@public.gmane.org>
> Cc: netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Cc: linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Signed-off-by: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
>
> Conflicts:
>
> drivers/net/mlx4/cmd.c
> drivers/net/mlx4/mlx4.h
> ---
> drivers/infiniband/hw/mthca/mthca_cmd.c | 12 ++++++++----
> drivers/infiniband/hw/mthca/mthca_dev.h | 3 ++-
> drivers/net/ethernet/mellanox/mlx4/cmd.c | 12 ++++++++----
> drivers/net/ethernet/mellanox/mlx4/mlx4.h | 3 ++-
> 4 files changed, 20 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/infiniband/hw/mthca/mthca_cmd.c b/drivers/infiniband/hw/mthca/mthca_cmd.c
> index 9d3e5c1ac60e..aad1852e8e10 100644
> --- a/drivers/infiniband/hw/mthca/mthca_cmd.c
> +++ b/drivers/infiniband/hw/mthca/mthca_cmd.c
> @@ -417,7 +417,8 @@ static int mthca_cmd_wait(struct mthca_dev *dev,
> int err = 0;
> struct mthca_cmd_context *context;
>
> - down(&dev->cmd.event_sem);
> + wait_event(dev->cmd.event_wait,
> + atomic_add_unless(&dev->cmd.commands, -1, 0));
>
> spin_lock(&dev->cmd.context_lock);
> BUG_ON(dev->cmd.free_head < 0);
> @@ -459,7 +460,8 @@ out:
> dev->cmd.free_head = context - dev->cmd.context;
> spin_unlock(&dev->cmd.context_lock);
>
> - up(&dev->cmd.event_sem);
> + atomic_inc(&dev->cmd.commands);
> + wake_up(&dev->cmd.event_wait);
> return err;
> }
>
> @@ -571,7 +573,8 @@ int mthca_cmd_use_events(struct mthca_dev *dev)
> dev->cmd.context[dev->cmd.max_cmds - 1].next = -1;
> dev->cmd.free_head = 0;
>
> - sema_init(&dev->cmd.event_sem, dev->cmd.max_cmds);
> + init_waitqueue_head(&dev->cmd.event_wait);
> + atomic_set(&dev->cmd.commands, dev->cmd.max_cmds);
> spin_lock_init(&dev->cmd.context_lock);
>
> for (dev->cmd.token_mask = 1;
> @@ -597,7 +600,8 @@ void mthca_cmd_use_polling(struct mthca_dev *dev)
> dev->cmd.flags &= ~MTHCA_CMD_USE_EVENTS;
>
> for (i = 0; i < dev->cmd.max_cmds; ++i)
> - down(&dev->cmd.event_sem);
> + wait_event(dev->cmd.event_wait,
> + atomic_add_unless(&dev->cmd.commands, -1, 0));
>
> kfree(dev->cmd.context);
>
> diff --git a/drivers/infiniband/hw/mthca/mthca_dev.h b/drivers/infiniband/hw/mthca/mthca_dev.h
> index 7e6a6d64ad4e..3055f5c12ac8 100644
> --- a/drivers/infiniband/hw/mthca/mthca_dev.h
> +++ b/drivers/infiniband/hw/mthca/mthca_dev.h
> @@ -121,7 +121,8 @@ struct mthca_cmd {
> struct pci_pool *pool;
> struct mutex hcr_mutex;
> struct semaphore poll_sem;
> - struct semaphore event_sem;
> + wait_queue_head_t event_wait;
> + atomic_t commands;
> int max_cmds;
> spinlock_t context_lock;
> int free_head;
> diff --git a/drivers/net/ethernet/mellanox/mlx4/cmd.c b/drivers/net/ethernet/mellanox/mlx4/cmd.c
> index 78f5a1a0b8c8..60134a4245ef 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/cmd.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/cmd.c
> @@ -273,7 +273,8 @@ static int mlx4_cmd_wait(struct mlx4_dev *dev, u64 in_param, u64 *out_param,
> struct mlx4_cmd_context *context;
> int err = 0;
>
> - down(&cmd->event_sem);
> + wait_event(cmd->event_wait,
> + atomic_add_unless(&cmd->commands, -1, 0));
>
> spin_lock(&cmd->context_lock);
> BUG_ON(cmd->free_head < 0);
> @@ -305,7 +306,8 @@ out:
> cmd->free_head = context - cmd->context;
> spin_unlock(&cmd->context_lock);
>
> - up(&cmd->event_sem);
> + atomic_inc(&cmd->commands);
> + wake_up(&cmd->event_wait);
> return err;
> }
>
> @@ -380,7 +382,8 @@ int mlx4_cmd_use_events(struct mlx4_dev *dev)
> priv->cmd.context[priv->cmd.max_cmds - 1].next = -1;
> priv->cmd.free_head = 0;
>
> - sema_init(&priv->cmd.event_sem, priv->cmd.max_cmds);
> + init_waitqueue_head(&priv->cmd.event_wait);
> + atomic_set(&priv->cmd.commands, priv->cmd.max_cmds);
> spin_lock_init(&priv->cmd.context_lock);
>
> for (priv->cmd.token_mask = 1;
> @@ -407,7 +410,8 @@ void mlx4_cmd_use_polling(struct mlx4_dev *dev)
> priv->cmd.use_events = 0;
>
> for (i = 0; i < priv->cmd.max_cmds; ++i)
> - down(&priv->cmd.event_sem);
> + wait_event(priv->cmd.event_wait,
> + atomic_add_unless(&priv->cmd.commands, -1, 0));
>
> kfree(priv->cmd.context);
>
> diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4.h b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> index 5dfa68ffc11c..5a2c55d8ccfd 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> @@ -189,7 +189,8 @@ struct mlx4_cmd {
> void __iomem *hcr;
> struct mutex hcr_mutex;
> struct semaphore poll_sem;
> - struct semaphore event_sem;
> + wait_queue_head_t event_wait;
> + atomic_t commands;
> int max_cmds;
> spinlock_t context_lock;
> int free_head;
>
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH 02/25] IB/mthca, net/mlx4: remove counting semaphores
2015-10-28 2:45 ` [PATCH 02/25] IB/mthca, net/mlx4: remove counting semaphores Arnd Bergmann
[not found] ` <1446000373-1823620-3-git-send-email-arnd-r2nGTMty4D4@public.gmane.org>
@ 2015-10-29 7:41 ` Jack Morgenstein
1 sibling, 0 replies; 3+ messages in thread
From: Jack Morgenstein @ 2015-10-29 7:41 UTC (permalink / raw)
To: Arnd Bergmann
Cc: tglx, Roland Dreier, Eli Cohen, Yevgeny Petrilin, netdev,
linux-rdma, matanb, yishaih, ogerlitz, eranbe, talal
On Wed, 28 Oct 2015 03:45:50 +0100
Arnd Bergmann <arnd@arndb.de> wrote:
> As far as I can tell, there is a preexisting race condition
> regarding the cmd->use_events flag, which is not protected
> by any lock. When this flag is toggled while another command
> is being started, that command gets stuck until the mode is
> toggled back.
We fixed this issue in mellanox ofed in a manner that allowed keeping
the same counting mechanism. IMHO, this is preferable, rather than
totally changing the mechanism.
We will submit a similar patch to the upstream kernel shortly.
-Jack
net/mlx4: Switching between sending commands via polling and events may results in hung tasks
When switching between those methonds of sending commands, it's possbile that a
task will keep waiting for the polling sempahore, but may never be able to acquire it.
This is due to mlx4_cmd_use_events which "down"s the sempahore back to 0.
Reproducing it involves in sending commands while chaning between mlx4_cmd_use_polling
and mlx4_cmd_use_events.
Solving it by adding a read-write semaphore when switching between modes.
issue: 402565
Change-Id: I19f0d40dbb327c49b39a9abbcb2bb002b0279b0b
Signed-off-by: Matan Barak <matanb@mellanox.com>
---
drivers/net/ethernet/mellanox/mlx4/cmd.c | 23 +++++++++++++++++------
drivers/net/ethernet/mellanox/mlx4/mlx4.h | 2 ++
2 files changed, 19 insertions(+), 6 deletions(-)
diff --git a/drivers/net/ethernet/mellanox/mlx4/cmd.c b/drivers/net/ethernet/mellanox/mlx4/cmd.c
index def1338..f94a960 100644
--- a/drivers/net/ethernet/mellanox/mlx4/cmd.c
+++ b/drivers/net/ethernet/mellanox/mlx4/cmd.c
@@ -766,17 +766,23 @@ int __mlx4_cmd(struct mlx4_dev *dev, u64 in_param, u64 *out_param,
return mlx4_cmd_reset_flow(dev, op, op_modifier, -EIO);
if (!mlx4_is_mfunc(dev) || (native && mlx4_is_master(dev))) {
+ int ret;
+
if (dev->persist->state & MLX4_DEVICE_STATE_INTERNAL_ERROR)
return mlx4_internal_err_ret_value(dev, op,
op_modifier);
+ down_read(&mlx4_priv(dev)->cmd.switch_sem);
if (mlx4_priv(dev)->cmd.use_events)
- return mlx4_cmd_wait(dev, in_param, out_param,
- out_is_imm, in_modifier,
- op_modifier, op, timeout);
+ ret = mlx4_cmd_wait(dev, in_param, out_param,
+ out_is_imm, in_modifier,
+ op_modifier, op, timeout);
else
- return mlx4_cmd_poll(dev, in_param, out_param,
- out_is_imm, in_modifier,
- op_modifier, op, timeout);
+ ret = mlx4_cmd_poll(dev, in_param, out_param,
+ out_is_imm, in_modifier,
+ op_modifier, op, timeout);
+
+ up_read(&mlx4_priv(dev)->cmd.switch_sem);
+ return ret;
}
return mlx4_slave_cmd(dev, in_param, out_param, out_is_imm,
in_modifier, op_modifier, op, timeout);
@@ -2437,6 +2443,7 @@ int mlx4_cmd_init(struct mlx4_dev *dev)
int flags = 0;
if (!priv->cmd.initialized) {
+ init_rwsem(&priv->cmd.switch_sem);
mutex_init(&priv->cmd.slave_cmd_mutex);
sema_init(&priv->cmd.poll_sem, 1);
priv->cmd.use_events = 0;
@@ -2566,6 +2573,7 @@ int mlx4_cmd_use_events(struct mlx4_dev *dev)
if (!priv->cmd.context)
return -ENOMEM;
+ down_write(&priv->cmd.switch_sem);
for (i = 0; i < priv->cmd.max_cmds; ++i) {
priv->cmd.context[i].token = i;
priv->cmd.context[i].next = i + 1;
@@ -2590,6 +2598,7 @@ int mlx4_cmd_use_events(struct mlx4_dev *dev)
down(&priv->cmd.poll_sem);
priv->cmd.use_events = 1;
+ up_write(&priv->cmd.switch_sem);
return err;
}
@@ -2602,6 +2611,7 @@ void mlx4_cmd_use_polling(struct mlx4_dev *dev)
struct mlx4_priv *priv = mlx4_priv(dev);
int i;
+ down_write(&priv->cmd.switch_sem);
priv->cmd.use_events = 0;
for (i = 0; i < priv->cmd.max_cmds; ++i)
@@ -2610,6 +2620,7 @@ void mlx4_cmd_use_polling(struct mlx4_dev *dev)
kfree(priv->cmd.context);
up(&priv->cmd.poll_sem);
+ up_write(&priv->cmd.switch_sem);
}
struct mlx4_cmd_mailbox *mlx4_alloc_cmd_mailbox(struct mlx4_dev *dev)
diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4.h b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
index 6c58021..2f03e6e 100644
--- a/drivers/net/ethernet/mellanox/mlx4/mlx4.h
+++ b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
@@ -45,6 +45,7 @@
#include <linux/workqueue.h>
#include <linux/interrupt.h>
#include <linux/spinlock.h>
+#include <linux/rwsem.h>
#include <linux/mlx4/device.h>
#include <linux/mlx4/driver.h>
@@ -626,6 +627,7 @@ struct mlx4_cmd {
struct mutex slave_cmd_mutex;
struct semaphore poll_sem;
struct semaphore event_sem;
+ struct rw_semaphore switch_sem;
int max_cmds;
spinlock_t context_lock;
int free_head;
--
1.8.1.2
^ permalink raw reply related [flat|nested] 3+ messages in thread
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[not found] <1446000373-1823620-1-git-send-email-arnd@arndb.de>
2015-10-28 2:45 ` [PATCH 02/25] IB/mthca, net/mlx4: remove counting semaphores Arnd Bergmann
[not found] ` <1446000373-1823620-3-git-send-email-arnd-r2nGTMty4D4@public.gmane.org>
2015-10-28 9:19 ` Sagi Grimberg
2015-10-29 7:41 ` Jack Morgenstein
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