Re: [PATCH v7 01/14] alpha/perf: do not update an event that has no counter in alpha_pmu_del()

[sashiko-bot@kernel.org]

> alpha_pmu_add() puts a new event on the list with current_idx set to
> PMC_NO_INDEX, and alpha_pmu_enable() assigns the counter later, through
> maybe_change_configuration(). Alpha has no group transaction callbacks,
> so the core schedules a group with the PMU disabled for the whole
> group. When a sibling does not fit, for example because the counters
> are multiplexed between groups, group_sched_in() rolls back the events
> it already added by calling alpha_pmu_del() on them, before the PMU is
> enabled again.
> 
> alpha_pmu_del() then passes idx == -1 to alpha_perf_event_update(),
> which indexes pmc_count_shift[], pmc_count_mask[] and pmc_max_period[]
> at -1, and it clears bit -1 of idx_mask, an undefined shift. On the
> EV67 PMU the entries before each array happen to make the computed
> delta zero, so the event's count survives, but only by luck of the
> struct layout.
> 
> The event was never on a counter, so there is no count to absorb and no
> counter to turn off. Only remove it from the list.
> 
> Fixes: 979f867191f8 ("alpha: implement HW performance events on the EV67 and later CPUs")
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Matt Turner <mattst88@gmail.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261105-alpha-profileme-v7-0-c474e892abb0@gmail.com?part=1