On Tue, 15 Oct 2013 15:58:06 -0400 Steven Rostedt <rost...@goodmis.org> 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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