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Date: Wed, 28 Oct 2015 11:19:20 +0200
From: Sagi Grimberg <sagig@....mellanox.co.il>
To: Arnd Bergmann <arnd@...db.de>, tglx@...utronix.de
Cc: Roland Dreier <roland@...estorage.com>,
Eli Cohen <eli@...lanox.co.il>,
Yevgeny Petrilin <yevgenyp@...lanox.co.il>,
netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Subject: Re: [PATCH 02/25] IB/mthca, net/mlx4: remove counting semaphores
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@...estorage.com>
> Cc: Eli Cohen <eli@...lanox.co.il>
> Cc: Yevgeny Petrilin <yevgenyp@...lanox.co.il>
> Cc: netdev@...r.kernel.org
> Cc: linux-rdma@...r.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.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;
>
--
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