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SubjectRe: [PATCH v2] ipc/msg: Implement lockless pipelined wakeups
On Tue, Nov 03, 2015 at 04:03:29PM +0100, Sebastian Andrzej Siewior wrote:

> - struct msg_msg *volatile r_msg;
> + struct msg_msg *r_msg;

> + wake_q_add(wake_q, msr->r_tsk);
> msr->r_msg = msg;
> + /*
> + * Rely on the implicit cmpxchg barrier from
> + * wake_q_add such that we can ensure that

Davidlohr, didn't you want to make that cmpxchg_relaxed() or
cmpxchg_release() ?

> + * updating msr->r_msg is the last write
> + * operation: As once set, the receiver can
> + * continue, and if we don't have the reference

Which seems to suggest you want to at least make that WRITE_ONCE()

> + * count from the wake_q, yet, at that point we
> + * can later have a use-after-free condition and
> + * bogus wakeup.
> + */

And I agree with DAvidlohr that the comment is placed oddly, one would
expect it between wake_q_add() and the store in question.




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