Messages in this thread Patch in this message | | | Date | Wed, 27 Jun 2018 22:10:28 -0700 | From | Joel Fernandes <> | Subject | Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq_snap works |
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On Wed, Jun 27, 2018 at 11:27:26AM -0700, Joel Fernandes wrote: [..] > > > > s = __ALIGN_MASK(s, RCU_SEQ_STATE_MASK); > > > > > > > > > > I agree with Peter's suggestions for both the verbiage reduction in the > > > comments in the header, as the new code he is proposing is more > > > self-documenting. I believe I proposed a big comment just because the code > > > wasn't self-documenting or obvious previously so needed an explanation. > > > > > > How would you like to proceed? Let me know what you guys decide, I am really > > > Ok with anything. If you guys agree, should I write a follow-up patch with > > > Peter's suggestion that applies on top of this one? Or do we want to drop > > > this one in favor of Peter's suggestion? > > > > Shortening the comment would be good, so please do that.
Paul,
Do you want to fold the below patch into the original one? Or do you prefer I resent the original patch fixed up?
Following is the patch ontop of current 'dev' branch in your tree, with the excessive comments removed.
Thanks to Peter for suggesting!
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From: "Joel Fernandes (Google)" <joel@joelfernandes.org> Subject: [PATCH] rcu: Remove excessive commentary on rcu_seq_snap
There isn't a strong need to explain in excessive detail about rcu_seq_snap with an example. Remove unnecessary and redundant comments.
Suggested-by: Peter Zijlstra <peterz@infradead.org> Fixes: 9701945dd79e ("rcu: Add comment documenting how rcu_seq_snap works") Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> --- kernel/rcu/rcu.h | 22 ---------------------- 1 file changed, 22 deletions(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 0af6ce6d8b66..4d04683c31b2 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -101,28 +101,6 @@ static inline void rcu_seq_end(unsigned long *sp) * 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) { -- 2.18.0.rc2.346.g013aa6912e-goog
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