Messages in this thread | | | Date | Thu, 23 Apr 2009 18:36:24 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier |
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On 04/22, Ingo Molnar wrote: > > * David Howells <dhowells@redhat.com> wrote: > > The dangerous pattern is lockless code doing wakeups. But lockless > code always has to use proper barriers or atomics anyway, and has to > be aware of the fact that kernel primitives they call are not > necessarily full memory barriers. > > In fact i'd encourage to _not_ document try_to_lock() as a write > barrier either - but rather have explicit barriers where they are > needed. Then we could remove that barrier from try_to_wake_up() too > ;-)
Well. I think in that case almost every try_to_wake_up/wake_up_process needs mb().
For example:
do_nanosleep:
for (;;) { set_current_state(TASK_INTERRUPTIBLE); if (likely(t->task)) schedule(); break; }
hrtimer_wakeup:
task = t->task t->task = NULL; wake_up_process(task);
If try_to_wake_up() has no the barrier semantics, we can miss the event. "t->task = NULL" and the reading of task->state must not be reordered.
Oleg.
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