[PATCH v2] powerpc/pseries/vas: Migration suspend waits for no in-progress open windows

[Nathan Lynch]

Haren Myneni <haren at linux.ibm.com> writes:
> The hypervisor returns migration failure if all VAS windows are not
> closed. During pre-migration stage, vas_migration_handler() sets
> migration_in_progress flag and closes all windows from the list.
> The allocate VAS window routine checks the migration flag, setup
> the window and then add it to the list. So there is possibility of
> the migration handler missing the window that is still in the
> process of setup.
>
> t1: Allocate and open VAS	t2: Migration event
>     window
>
> lock vas_pseries_mutex
> If migration_in_progress set
>   unlock vas_pseries_mutex
>   return
> open window HCALL
> unlock vas_pseries_mutex
> Modify window HCALL		lock vas_pseries_mutex
> setup window			migration_in_progress=true
> 				Closes all windows from
> 				the list
> 				unlock vas_pseries_mutex
> lock vas_pseries_mutex		return
> if nr_closed_windows == 0
>   // No DLPAR CPU or migration
>   add to the list
>   unlock vas_pseries_mutex
>   return
> unlock vas_pseries_mutex
> Close VAS window
> // due to DLPAR CPU or migration
> return -EBUSY
>
> This patch resolves the issue with the following steps:
> - Define migration_in_progress as atomic so that the migration
>   handler sets this flag without holding mutex.

This part of the commit message is no longer accurate...

> - Introduce nr_open_wins_progress counter in VAS capabilities
>   struct
> - This counter tracks the number of open windows are still in
>   progress
> - The allocate setup window thread closes windows if the migration
>   is set and decrements nr_open_window_progress counter
> - The migration handler waits for no in-progress open windows.
>
> Fixes: 37e6764895ef ("powerpc/pseries/vas: Add VAS migration handler")
> Signed-off-by: Haren Myneni <haren at linux.ibm.com>
>
> ---
> Changes from v1:
> - Do not define the migration_in_progress flag as atomic as
>   suggested by Nathan
> ---
>  arch/powerpc/platforms/pseries/vas.c | 45 +++++++++++++++++++++++-----
>  arch/powerpc/platforms/pseries/vas.h |  2 ++
>  2 files changed, 40 insertions(+), 7 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/vas.c b/arch/powerpc/platforms/pseries/vas.c
> index 15d958e38eca..b86f0db08e98 100644
> --- a/arch/powerpc/platforms/pseries/vas.c
> +++ b/arch/powerpc/platforms/pseries/vas.c
> @@ -32,6 +32,7 @@ static struct hv_vas_cop_feat_caps hv_cop_caps;
>  static struct vas_caps vascaps[VAS_MAX_FEAT_TYPE];
>  static DEFINE_MUTEX(vas_pseries_mutex);
>  static bool migration_in_progress;
> +static DECLARE_WAIT_QUEUE_HEAD(open_win_progress_wq);
>  
>  static long hcall_return_busy_check(long rc)
>  {
> @@ -384,11 +385,15 @@ static struct vas_window *vas_allocate_window(int vas_id, u64 flags,
>  	 * same fault IRQ is not freed by the OS before.
>  	 */
>  	mutex_lock(&vas_pseries_mutex);
> -	if (migration_in_progress)
> +	if (migration_in_progress) {
>  		rc = -EBUSY;
> -	else
> +	} else {
>  		rc = allocate_setup_window(txwin, (u64 *)&domain[0],
>  				   cop_feat_caps->win_type);
> +		if (!rc)
> +			atomic_inc(&caps->nr_open_wins_progress);
> +	}
> +
>  	mutex_unlock(&vas_pseries_mutex);
>  	if (rc)
>  		goto out;
> @@ -403,8 +408,17 @@ static struct vas_window *vas_allocate_window(int vas_id, u64 flags,
>  		goto out_free;
>  
>  	txwin->win_type = cop_feat_caps->win_type;
> -	mutex_lock(&vas_pseries_mutex);
> +
>  	/*
> +	 * The migration SUSPEND thread sets migration_in_progress and
> +	 * closes all open windows from the list. But the window is
> +	 * added to the list after open and modify HCALLs. So possible
> +	 * that migration_in_progress is set before modify HCALL which
> +	 * may cause some windows are still open when the hypervisor
> +	 * initiates the migration.
> +	 * So checks the migration_in_progress flag again and close all
> +	 * open windows.
> +	 *
>  	 * Possible to lose the acquired credit with DLPAR core
>  	 * removal after the window is opened. So if there are any
>  	 * closed windows (means with lost credits), do not give new
> @@ -412,9 +426,11 @@ static struct vas_window *vas_allocate_window(int vas_id, u64 flags,
>  	 * after the existing windows are reopened when credits are
>  	 * available.
>  	 */
> -	if (!caps->nr_close_wins) {
> +	mutex_lock(&vas_pseries_mutex);
> +	if (!caps->nr_close_wins && !migration_in_progress) {
>  		list_add(&txwin->win_list, &caps->list);
>  		caps->nr_open_windows++;
> +		atomic_dec(&caps->nr_open_wins_progress);

Should there not be a test and wakeup here?

if (atomic_dec_return(&caps->nr_open_wins_progress) == 0)
	wake_up(&open_win_progress_wq);

>  		mutex_unlock(&vas_pseries_mutex);
>  		vas_user_win_add_mm_context(&txwin->vas_win.task_ref);
>  		return &txwin->vas_win;
> @@ -432,6 +448,8 @@ static struct vas_window *vas_allocate_window(int vas_id, u64 flags,
>  	 */
>  	free_irq_setup(txwin);
>  	h_deallocate_vas_window(txwin->vas_win.winid);
> +	atomic_dec(&caps->nr_open_wins_progress);
> +	wake_up(&open_win_progress_wq);

I wonder if this should also wake only when the counter reaches zero.

>  out:
>  	atomic_dec(&cop_feat_caps->nr_used_credits);
>  	kfree(txwin);
> @@ -936,14 +954,14 @@ int vas_migration_handler(int action)
>  	struct vas_caps *vcaps;
>  	int i, rc = 0;
>  
> +	pr_info("VAS migration event %d\n", action);
> +

A leftover debug statement? It doesn't seem necessary.

>  	/*
>  	 * NX-GZIP is not enabled. Nothing to do for migration.
>  	 */
>  	if (!copypaste_feat)
>  		return rc;
>  
> -	mutex_lock(&vas_pseries_mutex);
> -
>  	if (action == VAS_SUSPEND)
>  		migration_in_progress = true;
>  	else
> @@ -989,12 +1007,24 @@ int vas_migration_handler(int action)
>  
>  		switch (action) {
>  		case VAS_SUSPEND:
> +			mutex_lock(&vas_pseries_mutex);
>  			rc = reconfig_close_windows(vcaps, vcaps->nr_open_windows,
>  							true);
> +			mutex_unlock(&vas_pseries_mutex);
> +			/*
> +			 * Windows are included in the list after successful
> +			 * open. So wait for closing these in-progress open
> +			 * windows in vas_allocate_window() which will be
> +			 * done if the migration_in_progress is set.
> +			 */
> +			rc = wait_event_interruptible(open_win_progress_wq,
> +				!atomic_read(&vcaps->nr_open_wins_progress));

I don't think the wait should be interruptible. We don't want a signal
to cause this function to return before all the windows are released.

Note that since vas_migration_handler() updates migration_in_progress
before acquiring the mutex and loads vcaps->nr_open_wins_progress after
releasing the mutex, this order of operations *could* occur:

  // when action == VAS_SUSPEND
  acquire vas_pseries_mutex
  load &vcaps->nr_open_wins_progress
  store &migration_in_progress
  release vas_pseries_mutex

Or at least I suspect so when reconfig_close_windows() has no work to
do. I'm relying on this passage from memory-barriers.txt:

  An ACQUIRE followed by a RELEASE may not be assumed to be full memory barrier
  because it is possible for an access preceding the ACQUIRE to happen after the
  ACQUIRE, and an access following the RELEASE to happen before the RELEASE, and
  the two accesses can themselves then cross:

	*A = a;
	ACQUIRE M
	RELEASE M
	*B = b;

  may occur as:

	ACQUIRE M, STORE *B, STORE *A, RELEASE M


Now, I don't currently see problems with this in the allocation
path. But the fact that reviewers now need to consider such
possibilities makes me want a different locking model in this code.

>  			break;
>  		case VAS_RESUME:
> +			mutex_lock(&vas_pseries_mutex);
>  			atomic_set(&caps->nr_total_credits, new_nr_creds);
>  			rc = reconfig_open_windows(vcaps, new_nr_creds, true);
> +			mutex_unlock(&vas_pseries_mutex);
>  			break;
>  		default:
>  			/* should not happen */
> @@ -1010,8 +1040,9 @@ int vas_migration_handler(int action)
>  			goto out;
>  	}
>  
> +	pr_info("VAS migration event (%d) successful\n", action);
> +

Looks like another leftover debug statement.

>  out:
> -	mutex_unlock(&vas_pseries_mutex);
>  	return rc;
>  }