On 13-Oct-22 2:17 AM, Peter Zijlstra wrote:

On Wed, Oct 12, 2022 at 02:16:29PM +0200, Peter Zijlstra wrote:

quoted

That's the intent yeah. But due to not always holding ctx->mutex over
put_pmu_ctx() this might be moot. I'm almost through auditing epc usage
and I think ctx->lock is sufficient, fingers crossed.

So the very last epc usage threw a spanner into the works and made
things complicated.

Specifically sys_perf_event_open()'s group_leader case uses
event->pmu_ctx while only holding ctx->mutex. Therefore we can't fully
let go of ctx->mutex locking and purely rely on ctx->lock.

Now the good news is that the annoying put_pmu_ctx() without holding
ctx->mutex case doesn't actually matter here. Since we hold a reference
on the group_leader (per the filedesc) the event can't go away,
therefore it must have a pmu_ctx, and then holding ctx->mutex ensures
the pmu_ctx is stable -- iow it serializes against
sys_perf_event_open()'s move_group and perf_pmu_migrate_context()
changing the epc around.

So we're going with the normal mutex+lock for modification rule, but
allow the weird put_pmu_ctx() exception.

I have the below delta.

I'm hoping we can call this done -- I'm going to see if I can bribe Mark
to take a look at the arm64 thing soon and then hopefully queue the
whole thing once -rc1 happens. That should give us a good long soak
until the next merge window.

Sounds good. Thanks for all the help!

I've glanced through the changes and they looks fine, below are few minor
points.

+ * Specificially, sys_perf_event_open()'s group_leader case depends on
+ * ctx->mutex pinning the configuration. Since we hold a reference on
+ * group_leader (through the filedesc) it can't fo away, therefore it's

typo: can't go away

-	refcount_t			refcount;
+	refcount_t			refcount; /* event <-> ctx */

Ok. We need to remove all those // XXX get/put_ctx() from code
which we added to make refcount a pmu_ctx <-> ctx.

+#define double_list_for_each_entry(pos1, pos2, head1, head2, member)	\
+	for (pos1 = list_first_entry(head1, typeof(*pos1), member),	\
+	     pos2 = list_first_entry(head2, typeof(*pos2), member);	\
+	     !list_entry_is_head(pos1, head1, member) &&		\
+	     !list_entry_is_head(pos2, head2, member);			\
+	     pos1 = list_next_entry(pos1, member),			\
+	     pos2 = list_next_entry(pos2, member))
+
 static void perf_event_swap_task_ctx_data(struct perf_event_context *prev_ctx,
 					  struct perf_event_context *next_ctx)

While this is unrelated to this patch, shouldn't we also need to swap
event->hw.target? A purely hypothetical scenario: Consider two processes
having clone contexts (for example, two children of the same parent).
While process switch between these two, the perf event context would get
swapped but event->hw.target will point to other sibling's task_struct.
If any one process exit just after single context swap, _free_event()
will call put_task_context() on sibling process' task_struct.

typo: perf_event_pmu_context

The good (or bad? ;)) news is, perf test and Vince's perf_event_tests
are running fine without any regression on my machine.

Thanks,
Ravi