Re: [PATCH] bug: Use xchg() to update WARN_ON_ONCE() staticvariable

[Andrew Morton]

On Tue, 15 Oct 2013 15:58:06 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> The WARN_ON_ONCE() code is to trigger a waring only once when some
> condition happens. But due to the way it is written it is racy.
> 
> 	if (unlikely(condition)) {
> 		if (WARN(!__warned))
> 			__warned = true;
> 	}
> 
> The problem is that multiple CPUs could hit the same warning and
> produce multiple output dumps of the same warning, or an interrupt could
> happen and hit the same warning and do the warning in the middle of a
> previous one, especially since the WARN() does a dump of the current
> stack.
> 
> Even more of a problem, a recent WARN_ON_ONCE() that was in the page
> fault handler triggered and the stack dump of the WARN() caused the
> same WARN_ON_ONCE() get hit again. Since the __warned = true is not
> updated until after the WARN() is completed, each WARN() triggered
> another page fault causing the stack to be filled and crashed the box.
> 
> The point of WARN_ON() is to warn the user and not to crash the box.
> 
> The easy fix is to update the __warned variable with a xchg(). This way
> only one WARN_ON_ONCE() will actually happen, and prevents any issues
> of the WARN() causing the same WARN() to be hit and crash the system.

printk_once() has the same issue, and probably other places.

Is there some sneaky way of doing this operation as a common thing,
rather than open-coding it everywhere?  Something like

#define ONCE() ({
	static int state;
	int ret;

	ret = !xchg(&state, 1);
	ret;
})

Also, is xchg() better than test_and_set_bit()?  (test_and_set_bit()
requires a long, so more storage).

Also, we're now incurring an atomic op for every "call".  Presumably
these calls are rare, but not necessarily - one can envisage uses of a
generic ONCE() which are called at high frequency.  Should we avoid
that with

#define ONCE() ({
	static int state;
	int ret;

	if (likely(state))
		ret = 0;
	else
		ret = !xchg(&state, 1);
	ret;
})

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