Messages in this thread Patch in this message | | | Date | Sun, 20 May 2018 22:48:26 -0700 | From | Joel Fernandes <> | Subject | Re: [PATCH v3 1/4] rcu: Add comment documenting how rcu_seq_snap works |
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On Sun, May 20, 2018 at 09:50:25PM -0700, Randy Dunlap wrote: > On 05/20/2018 09:42 PM, Joel Fernandes wrote: > > 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> > > --- > > kernel/rcu/rcu.h | 33 ++++++++++++++++++++++++++++++++- > > 1 file changed, 32 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > > index 0453a7d12b3f..d4396c96f614 100644 > > --- a/kernel/rcu/rcu.h > > +++ b/kernel/rcu/rcu.h > > @@ -91,7 +91,38 @@ 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. > > + * > > + * For example, since 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, its sufficient if we > > it's > > > + * 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. Incase the lower bit was set, the overflow > > In case > > > + * 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 incase the > > in case > > > + * overflow didn't occur. This masking is needed because incase RCU was idle > > in case > > > + * (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; > > > > cheers. > -- > ~Randy
Thanks Randy. Fixed, updated patch below. Paul, let me know if you want me to send it separately or if you can pick it up from below.
Also I realize I need some better automated tools to catch these issues (spelling errors in commit, diffs etc). Probably checkpatch.pl should have such checks for these common things too.
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From 1c1f8ce04bca656a3c07e555048545d4a59e44cf Mon Sep 17 00:00:00 2001 From: Joel Fernandes <joel@joelfernandes.org> Date: Sun, 20 May 2018 19:37:18 -0700 Subject: [PATCH v3.5] rcu: Add comment documenting how rcu_seq_snap works
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> --- kernel/rcu/rcu.h | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 0453a7d12b3f..00df3da98317 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -91,7 +91,38 @@ 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. + * + * For example, since 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, its 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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