netdev - Re: [PATCH V4 mlx5-next 13/13] vfio/mlx5: Use its own PCI reset

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Message-ID: <20211026171644.41019161.alex.williamson@redhat.com>
Date:   Tue, 26 Oct 2021 17:16:44 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Yishai Hadas <yishaih@...dia.com>
Cc:     <bhelgaas@...gle.com>, <jgg@...dia.com>, <saeedm@...dia.com>,
        <linux-pci@...r.kernel.org>, <kvm@...r.kernel.org>,
        <netdev@...r.kernel.org>, <kuba@...nel.org>, <leonro@...dia.com>,
        <kwankhede@...dia.com>, <mgurtovoy@...dia.com>, <maorg@...dia.com>
Subject: Re: [PATCH V4 mlx5-next 13/13] vfio/mlx5: Use its own PCI
 reset_done error handler

On Tue, 26 Oct 2021 12:06:05 +0300
Yishai Hadas <yishaih@...dia.com> wrote:

> Register its own handler for pci_error_handlers.reset_done and update
> state accordingly.
> 
> Signed-off-by: Yishai Hadas <yishaih@...dia.com>
> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> ---
>  drivers/vfio/pci/mlx5/main.c | 54 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 52 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vfio/pci/mlx5/main.c b/drivers/vfio/pci/mlx5/main.c
> index 4b21b388dcc5..c157f540d384 100644
> --- a/drivers/vfio/pci/mlx5/main.c
> +++ b/drivers/vfio/pci/mlx5/main.c
> @@ -55,8 +55,11 @@ struct mlx5vf_pci_migration_info {
>  struct mlx5vf_pci_core_device {
>  	struct vfio_pci_core_device core_device;
>  	u8 migrate_cap:1;
> +	u8 defered_reset:1;

s/defered/deferred/ throughout

>  	/* protect migration state */
>  	struct mutex state_mutex;
> +	/* protect the reset_done flow */
> +	spinlock_t reset_lock;
>  	struct mlx5vf_pci_migration_info vmig;
>  };
>  
> @@ -471,6 +474,47 @@ mlx5vf_pci_migration_data_rw(struct mlx5vf_pci_core_device *mvdev,
>  	return count;
>  }
>  
> +/* This function is called in all state_mutex unlock cases to
> + * handle a 'defered_reset' if exists.
> + */

I refrained from noting it elsewhere, but we're not in net/ or
drivers/net/ here, but we're using their multi-line comment style.  Are
we using the strong relation to a driver that does belong there as
justification for the style here?

> +static void mlx5vf_state_mutex_unlock(struct mlx5vf_pci_core_device *mvdev)
> +{
> +again:
> +	spin_lock(&mvdev->reset_lock);
> +	if (mvdev->defered_reset) {
> +		mvdev->defered_reset = false;
> +		spin_unlock(&mvdev->reset_lock);
> +		mlx5vf_reset_mig_state(mvdev);
> +		mvdev->vmig.vfio_dev_state = VFIO_DEVICE_STATE_RUNNING;
> +		goto again;
> +	}
> +	mutex_unlock(&mvdev->state_mutex);
> +	spin_unlock(&mvdev->reset_lock);
> +}
> +
> +static void mlx5vf_pci_aer_reset_done(struct pci_dev *pdev)
> +{
> +	struct mlx5vf_pci_core_device *mvdev = dev_get_drvdata(&pdev->dev);
> +
> +	if (!mvdev->migrate_cap)
> +		return;
> +
> +	/* As the higher VFIO layers are holding locks across reset and using
> +	 * those same locks with the mm_lock we need to prevent ABBA deadlock
> +	 * with the state_mutex and mm_lock.
> +	 * In case the state_mutex was taken alreday we differ the cleanup work

s/alreday/already/  s/differ/defer/ 

> +	 * to the unlock flow of the other running context.
> +	 */
> +	spin_lock(&mvdev->reset_lock);
> +	mvdev->defered_reset = true;
> +	if (!mutex_trylock(&mvdev->state_mutex)) {
> +		spin_unlock(&mvdev->reset_lock);
> +		return;
> +	}
> +	spin_unlock(&mvdev->reset_lock);
> +	mlx5vf_state_mutex_unlock(mvdev);
> +}
> +
>  static ssize_t mlx5vf_pci_mig_rw(struct vfio_pci_core_device *vdev,
>  				 char __user *buf, size_t count, loff_t *ppos,
>  				 bool iswrite)
> @@ -539,7 +583,7 @@ static ssize_t mlx5vf_pci_mig_rw(struct vfio_pci_core_device *vdev,
>  	}
>  
>  end:
> -	mutex_unlock(&mvdev->state_mutex);
> +	mlx5vf_state_mutex_unlock(mvdev);

I'm a little lost here, if the operation was to read the device_state
and mvdev->vmig.vfio_dev_state was error, that's already been copied to
the user buffer, so the user continues to see the error state for the
first read of device_state after reset if they encounter this race?
Thanks,

Alex

>  	return ret;
>  }
>  
> @@ -634,6 +678,7 @@ static int mlx5vf_pci_probe(struct pci_dev *pdev,
>  			if (MLX5_CAP_GEN(mdev, migration)) {
>  				mvdev->migrate_cap = 1;
>  				mutex_init(&mvdev->state_mutex);
> +				spin_lock_init(&mvdev->reset_lock);
>  			}
>  			mlx5_vf_put_core_dev(mdev);
>  		}
> @@ -668,12 +713,17 @@ static const struct pci_device_id mlx5vf_pci_table[] = {
>  
>  MODULE_DEVICE_TABLE(pci, mlx5vf_pci_table);
>  
> +const struct pci_error_handlers mlx5vf_err_handlers = {
> +	.reset_done = mlx5vf_pci_aer_reset_done,
> +	.error_detected = vfio_pci_aer_err_detected,
> +};
> +
>  static struct pci_driver mlx5vf_pci_driver = {
>  	.name = KBUILD_MODNAME,
>  	.id_table = mlx5vf_pci_table,
>  	.probe = mlx5vf_pci_probe,
>  	.remove = mlx5vf_pci_remove,
> -	.err_handler = &vfio_pci_core_err_handlers,
> +	.err_handler = &mlx5vf_err_handlers,
>  };
>  
>  static void __exit mlx5vf_pci_cleanup(void)