Messages in this thread | | | Date | Fri, 18 Sep 2015 15:44:53 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] kernel: fix data race in put_pid |
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On 09/18, Peter Zijlstra wrote: > > Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can > more conveniently use atomics in control dependencies. > > Since we can assume atomic_read() implies a READ_ONCE(), we must only > emit an extra smp_read_barrier_depends() in order to upgrade to > READ_ONCE_CTRL() semantics.
...
> +static inline int atomic_read_ctrl(atomic_t *v) > +{ > + int val = atomic_read(v); > + smp_read_barrier_depends(); /* Enforce control dependency. */ > + return val; > +}
Help. I am starting to think that the control dependencies is even more hard to understand that memory barriers...
So I assume that if we have
int X = 0; atomic_t Y = ATOMIC_INIT(0);
void w(void) { X = 1; atomic_inc_return(&Y); }
then
void r(void) { if (atomic_read_ctrl(&Y)) BUG_ON(X == 0); }
should be correct? Why?
If not then I am even more confused.
Oleg.
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