Messages in this thread | | | Date | Wed, 23 May 2018 09:04:00 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/4] rcu: Add comment documenting how rcu_seq_snap works |
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On Tue, May 22, 2018 at 11:38:13PM -0700, Joel Fernandes wrote: > From: "Joel Fernandes (Google)" <joel@joelfernandes.org> > > rcu_seq_snap may be tricky to decipher. Lets document how it works with > an example to make it easier. > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Very good, queued for further review, thank you!
Thanx, Paul
> --- > kernel/rcu/rcu.h | 34 +++++++++++++++++++++++++++++++++- > 1 file changed, 33 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > index 0453a7d12b3f..5819a37f19fb 100644 > --- a/kernel/rcu/rcu.h > +++ b/kernel/rcu/rcu.h > @@ -91,7 +91,39 @@ static inline void rcu_seq_end(unsigned long *sp) > WRITE_ONCE(*sp, rcu_seq_endval(sp)); > } > > -/* Take a snapshot of the update side's sequence number. */ > +/* > + * rcu_seq_snap - Take a snapshot of the update side's sequence number. > + * > + * This function returns the earliest value of the grace-period sequence number > + * that will indicate that a full grace period has elapsed since the current > + * time. Once the grace-period sequence number has reached this value, it will > + * be safe to invoke all callbacks that have been registered prior to the > + * current time. This value is the current grace-period number plus two to the > + * power of the number of low-order bits reserved for state, then rounded up to > + * the next value in which the state bits are all zero. > + * > + * In the current design, RCU_SEQ_STATE_MASK=3 and the least significant bit of > + * the seq is used to track if a GP is in progress or not. Given this, it is > + * sufficient if we add (6+1) and mask with ~3 to get the next GP. Let's see > + * why with an example: > + * > + * Say the current seq is 12 which is 0b1100 (GP is 3 and state bits are 0b00). > + * To get to the next GP number of 4, we have to add 0b100 to this (0x1 << 2) > + * to account for the shift due to 2 state bits. Now, if the current seq is > + * 13 (GP is 3 and state bits are 0b01), then it means the current grace period > + * is already in progress so the next GP that a future call back will be queued > + * to run at is GP+2 = 5, not 4. To account for the extra +1, we just overflow > + * the 2 lower bits by adding 0b11. In case the lower bit was set, the overflow > + * will cause the extra +1 to the GP, along with the usual +1 explained before. > + * This gives us GP+2. Finally we mask the lower to bits by ~0x3 in case the > + * overflow didn't occur. This masking is needed because in case RCU was idle > + * (no GP in progress so lower 2 bits are 0b00), then the overflow of the lower > + * 2 state bits wouldn't occur, so we mask to zero out those lower 2 bits. > + * > + * In other words, the next seq can be obtained by (0b11 + 0b100) & (~0b11) > + * which can be generalized to: > + * seq + (RCU_SEQ_STATE_MASK + (RCU_SEQ_STATE_MASK + 1)) & (~RCU_SEQ_STATE_MASK) > + */ > static inline unsigned long rcu_seq_snap(unsigned long *sp) > { > unsigned long s; > -- > 2.17.0.441.gb46fe60e1d-goog >
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