Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

[Frederic Weisbecker]

On Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen wrote:
> + * Protection of the tmigr group state information:
> + * ------------------------------------------------
> + *
> + * The state information with the list of active children and migrator needs to
> + * be protected by a sequence counter. It prevents a race when updates in child
> + * groups are propagated in changed order. The state update is performed
> + * lockless and group wise. The following scenario describes what happens
> + * without updating the sequence counter:
> + *
> + * Therefore, let's take three groups and four CPUs (CPU2 and CPU3 as well
> + * as GRP0:1 will not change during the scenario):
> + *
> + *    LVL 1            [GRP1:0]
> + *                     migrator = GRP0:1
> + *                     active   = GRP0:0, GRP0:1
> + *                   /                \
> + *    LVL 0  [GRP0:0]                  [GRP0:1]
> + *           migrator = CPU0           migrator = CPU2
> + *           active   = CPU0           active   = CPU2
> + *              /         \                /         \
> + *    CPUs     0           1              2           3
> + *             active      idle           active      idle
> + *
> + *
> + * 1. CPU0 goes idle. As the update is performed group wise, in the first step
> + *    only GRP0:0 is updated. The update of GRP1:0 is pending as CPU0 has to
> + *    walk the hierarchy.
> + *
> + *    LVL 1            [GRP1:0]
> + *                     migrator = GRP0:1
> + *                     active   = GRP0:0, GRP0:1
> + *                   /                \
> + *    LVL 0  [GRP0:0]                  [GRP0:1]
> + *       --> migrator = TMIGR_NONE     migrator = CPU2
> + *       --> active   =                active   = CPU2
> + *              /         \                /         \
> + *    CPUs     0           1              2           3
> + *         --> idle        idle           active      idle
> + *
> + * 2. While CPU0 goes idle and continues to update the state, CPU1 comes out of
> + *    idle. CPU1 updates GRP0:0. The update for GRP1:0 is pending as CPU1 also
> + *    has to the hierarchy. Both CPUs (CPU0 and CPU1) now walk the hierarchy to
> + *    perform the needed update from their point of view. The currently visible
> + *    state looks the following:
> + *
> + *    LVL 1            [GRP1:0]
> + *                     migrator = GRP0:1
> + *                     active   = GRP0:0, GRP0:1
> + *                   /                \
> + *    LVL 0  [GRP0:0]                  [GRP0:1]
> + *       --> migrator = CPU1           migrator = CPU2
> + *       --> active   = CPU1           active   = CPU2
> + *              /         \                /         \
> + *    CPUs     0           1              2           3
> + *             idle    --> active         active      idle
> + *
> + * 3. Here is the race condition: CPU1 managed to propagate its changes (from
> + *    step 2) through the hierarchy to GRP1:0 before CPU0 (step 1) did. The
> + *    active members of GRP1:0 remain unchanged after the update since it is
> + *    still valid from CPU1 current point of view:
> + *
> + *    LVL 1            [GRP1:0]
> + *                 --> migrator = GRP0:1
> + *                 --> active   = GRP0:0, GRP0:1
> + *                   /                \
> + *    LVL 0  [GRP0:0]                  [GRP0:1]
> + *           migrator = CPU1           migrator = CPU2
> + *           active   = CPU1           active   = CPU2
> + *              /         \                /         \
> + *    CPUs     0           1              2           3
> + *             idle        active         active      idle

So let's take this scenario and suppose we are at this stage. CPU 1
has propagated the state up to [GRP1:0] and CPU 0 is going to do it
but hasn't yet.

> +static bool tmigr_inactive_up(struct tmigr_group *group,
> +			      struct tmigr_group *child,
> +			      void *ptr)
> +{
> +	union tmigr_state curstate, newstate, childstate;
> +	struct tmigr_walk *data = ptr;
> +	bool walk_done;
> +	u8 childmask;
> +
> +	childmask = data->childmask;
> +	curstate.state = atomic_read(&group->migr_state);

And now suppose CPU 0 arrives here and sees the group->migr_state change
performed by CPU 1. So it's all good, right? The below atomic_cmpxchg()
will success on the first take.

> +	childstate.state = 0;
> +
> +	do {
> +		if (child)
> +			childstate.state = atomic_read(&child->migr_state);

But then how do you guarantee that CPU 0 will load here the version of
child->migr_state modified by CPU 1? What prevents from loading the stale value?
The one that was modified by CPU 0 instead? Nothing because the two above reads
are unordered. As a result, CPU 0 may ignore the fact that CPU 1 is up and
wrongly report GRP0:0 as active up to GRP1:0.

One way to solve this is to change the above atomic_read(&group->migr_state)
into atomic_read_acquire(&group->migr_state). It's cheap and pairs with the
order enforced by the upwards successful cmpxchg calls.

> +
> +		newstate = curstate;
> +		walk_done = true;
> +
> +		/* Reset active bit when the child is no longer active */
> +		if (!childstate.active)
> +			newstate.active &= ~childmask;
> +
> +		if (newstate.migrator == childmask) {
> +			/*
> +			 * Find a new migrator for the group, because the child
> +			 * group is idle!
> +			 */
> +			if (!childstate.active) {
> +				unsigned long new_migr_bit, active = newstate.active;
> +
> +				new_migr_bit = find_first_bit(&active, BIT_CNT);
> +
> +				if (new_migr_bit != BIT_CNT) {
> +					newstate.migrator = BIT(new_migr_bit);
> +				} else {
> +					newstate.migrator = TMIGR_NONE;
> +
> +					/* Changes need to be propagated */
> +					walk_done = false;
> +				}
> +			}
> +		}
> +
> +		newstate.seq++;
> +
> +		WARN_ON_ONCE((newstate.migrator != TMIGR_NONE) && !(newstate.active));
> +
> +	} while (!atomic_try_cmpxchg(&group->migr_state, &curstate.state, newstate.state));

Similarly, I seem to remember that a failing cmpxchg() doesn't imply a full
memory barrier. If it's the case, you may need to reload &group->migr_state
using an acquire barrier. But lemme check that...

Thanks.