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Consider the following race:
--------
o OP_CTL contains stale value: OP_CTL[Val]=1, OP_CTL[En]=0
o A new IBS OP event is being added
o [P]: Process context, [N]: NMI context
[P] perf_ibs_add(event) {
[P] if (test_and_set_bit(IBS_ENABLED, pcpu->state))
[P] return;
[P] /* pcpu->state = IBS_ENABLED */
[P]
[P] pcpu->event = event;
[P]
[P] perf_ibs_start(event) {
[P] set_bit(IBS_STARTED, pcpu->state);
[P] /* pcpu->state = IBS_ENABLED | IBS_STARTED */
[P] clear_bit(IBS_STOPPING, pcpu->state);
[P] /* pcpu->state = IBS_ENABLED | IBS_STARTED */
[N] --> NMI due to genuine FETCH event. perf_ibs_handle_irq()
[N] called for OP PMU as well.
[N]
[N] perf_ibs_handle_irq(perf_ibs) {
[N] event = pcpu->event; /* See line 6 */
[N]
[N] if (!test_bit(IBS_STARTED, pcpu->state)) /* false */
[N] return 0;
[N]
[N] if (WARN_ON_ONCE(!event)) /* false */
[N] goto fail;
[N]
[N] if (!(*buf++ & perf_ibs->valid_mask)) /* false due to stale
[N] * IBS_OP_CTL value */
[N] goto fail;
[N]
[N] ...
[N]
[N] perf_ibs_enable_event() // *Accidentally* enable the event.
[N] }
[N]
[N] /*
[N] * Repeated NMIs may follow due to accidentally enabled IBS OP
[N] * event if the sample period is very low. It could also lead
[N] * to pcpu->state corruption if the event gets throttled due
[N] * to too frequent NMIs.
[N] */
[P] perf_ibs_enable_event();
[P] }
[P] }
--------
We cannot safely clear IBS_{FETCH|OP}_CTL while disabling the event,
because the register might be read again later. So, clear the register
in the enable path - before we update pcpu->state and enable the event.
This guarantees that any NMI that lands in the gap finds Val=0 and
bails out cleanly.
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://patch.msgid.link/20260216042216.1440-6-ravi.bangoria@amd.com
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