On Fri, 11 Oct 2019, Manfred Spraul wrote:
But you are right, there are two different scenarios:
1) thread already in another wake_q, wakeup happens immediately after
the cmpxchg_relaxed().
This scenario is safe, due to the smp_mb__before_atomic() in wake_q_add()
2) thread woken up but e.g. a timeout, see ->state=STATE_READY,
returns to user space, calls sys_exit.
This must not happen before get_task_struct acquired a reference.
And this appears to be unsafe: get_task_struct() is refcount_inc(),
which is refcount_inc_checked(), which is according to lib/refcount.c
fully unordered.
Thus: ->state=STATE_READY can execute before the refcount increase.
Thus: ->state=STATE_READY needs a smp_store_release(), correct?
What if we did the reference count explicitly, and then just use
wake_q_add_safe()? That would avoid the extra barrier, __pipelined_op()
would become:
list_del();
get_task_struct();
wake_q_add_safe();
WRITE_ONCE(->state, STATE_READY);
Thanks,
Davidlohr
