linux-kernel - Re: [PATCH v3 56/57] perf: Simplify perf_pmu_output

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Date:   Mon, 12 Jun 2023 20:55:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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Subject: Re: [PATCH v3 56/57] perf: Simplify perf_pmu_output_stop()

On Mon, Jun 12, 2023 at 09:19:19AM -0700, Linus Torvalds wrote:
> This patch looks completely broken to me.
> 
> You now do
> 
>                 if (err == -EAGAIN)
>                         goto restart;
> 
> *within* the RCU-guarded section, and the "goto restart" will guard it again.

restart:
	guard(rcu)();
	list_for_each_entry_rcu(iter, &head, rb_entry) {
		...
		if (err == -EAGAIN)
			goto restart;
	}

So the restart is *before* the variable exists, eg. it's out-of-scope.

per the last email's guard.c, if changed like so:

void main(void)
{
	int done = 0;
restart:
	lock_guard(spin, moo, &lock);
	for (;!done;) {
		done = 1;
		goto restart;
	}
}

$ gcc -O2 -o guard guard.c && ./guard
spin_lock
spin_unlock
spin_lock
spin_unlock

Which is exactly the expected result.