Re: [PATCH v13 10/14] unwind: Clear unwind_mask on exit back to user space

On Tue, 15 Jul 2025 14:06:50 -0400
Steven Rostedt [off-list ref] wrote:

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+ * Return: 0 if the callback successfully was queued.
+ *         UNWIND_ALREADY_PENDING if the the callback was already queued.
+ *         UNWIND_ALREADY_EXECUTED if the callback was already called
+ *                (and will not be called again)
  *         Negative if there's an error.
  *         @cookie holds the cookie of the first request by any user
  */      

Lots of babbling in the Changelog, but no real elucidation as to why you
need this second return value.

AFAICT it serves no real purpose; the users of this function should not
care. The only difference is that the unwind reference (your cookie)
becomes a backward reference instead of a forward reference. But why
would anybody care?    

Older versions of the code required it. I think I can remove it now.  

Ah it is still used in the perf code:

perf_callchain() has:

        if (defer_user) {
                int ret = deferred_request(event);
                if (!ret)
                        local_inc(&event->ctx->nr_no_switch_fast);

Hmm, I guess this could work if it returned non zero for both already
queued and already executed. So it doesn't need to be two different
values.

-- Steve

                else if (ret < 0)
                        defer_user = false;
        }