At 2015/7/28 19:46, Paolo Bonzini Wrote:
On 28/07/2015 12:33, Wen Congyang wrote:
On 07/28/2015 06:18 PM, Paolo Bonzini wrote:
On 28/07/2015 12:02, Wen Congyang wrote:
I have a question about rcu: while do we call wait_for_readers()
twice for 32-bit host?
Because there is a very small but non-zero probability of the counter
going up by exactly 2^31 periods (periods are stored in bits 1-31 so you
lose one bit) while the thread is sleeping. This detail of the
implementation comes from URCU.
Yes, so you use rcu_gp_ctr ^ RCU_GP_CTR to instead of rcu_gp_ctr + RCU_GP_CTR.
The initial value is 1, so rcu_gp_ctr is: 1, 3, 1, 3, ...
The rcu_gp_ctr will never be 0. I think calling wait_for_readers() once is
enough.
Do I miss something?
If you call it just once, you have the same problem as before. In fact,
it's worse because instead of having an overflow every 2^31 periods, you
have one every 2 periods. Instead, by checking that rcu_reader went
through 1 _and_ 3 (or that it was at least once 0, i.e. the thread was
quiescent), you are sure that the thread went through _at least one_
grace period.
The overflow is acceptable. We only compare if rcu_reader.ctr is equal than
rcu_gp_ctr. If not, we should wait that thread to call rcu_read_unlock().
We don't care which is bigger. If no threads calls sync_rcu(), all threads
rcu_read.ctr is 0 or rcu_gp_ctr. If one thread calls sync_rcu(), all threads
rcu_read.ctr is 0, old_rcu_gp_ctr, or new_rcu_gp_ctr. We only wait the
thread
that's rcu_read.ctr is old_rcu_gp_ctr.
Thanks
Wen Congyang
Paolo
