• Paul E. McKenney's avatar
    rcu: Provide grace-period piggybacking API · 765a3f4f
    Paul E. McKenney authored
    The following pattern is currently not well supported by RCU:
    1.	Make data element inaccessible to RCU readers.
    2.	Do work that probably lasts for more than one grace period.
    3.	Do something to make sure RCU readers in flight before #1 above
    	have completed.
    Here are some things that could currently be done:
    a.	Do a synchronize_rcu() unconditionally at either #1 or #3 above.
    	This works, but imposes needless work and latency.
    b.	Post an RCU callback at #1 above that does a wakeup, then
    	wait for the wakeup at #3.  This works well, but likely results
    	in an extra unneeded grace period.  Open-coding this is also
    	a bit more semi-tricky code than would be good.
    This commit therefore adds get_state_synchronize_rcu() and
    cond_synchronize_rcu() APIs.  Call get_state_synchronize_rcu() at #1
    above and pass its return value to cond_synchronize_rcu() at #3 above.
    This results in a call to synchronize_rcu() if no grace period has
    elapsed between #1 and #3, but requires only a load, comparison, and
    memory barrier if a full grace period did elapse.
    Requested-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
rcutree.h 3.34 KB