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* [PATCH v2] rcu: Add comment documenting how rcu_seq_snap works
@ 2018-05-12  2:20 Joel Fernandes (Google)
  2018-05-12  2:38 ` Randy Dunlap
  0 siblings, 1 reply; 2+ messages in thread
From: Joel Fernandes (Google) @ 2018-05-12  2:20 UTC (permalink / raw)
  To: linux-kernel; +Cc: rdunlap, paulmck

rcu_seq_snap may be tricky for someone looking at it for the first time.
Lets document how it works with an example to make it easier.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
v2 changes: Corrections as suggested by Randy.

 kernel/rcu/rcu.h | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 003671825d62..533bc1087371 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -91,7 +91,29 @@ 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. */
+/*
+ * Take a snapshot of the update side's sequence number.
+ *
+ * This function predicts what the grace period number will be the next
+ * time an RCU callback will be executed, given the current grace period's
+ * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is
+ * already in progress.
+ *
+ * We do this with a single addition and masking.
+ * For example, if RCU_SEQ_STATE_MASK=1 and the least significant bit (LSB) of
+ * the seq is used to track if a GP is in progress or not, its sufficient if we
+ * add (2+1) and mask with ~1. Lets see why with an example:
+ *
+ * Say the current seq is 6 which is 0b110 (gp is 3 and state bit is 0).
+ * To get the next GP number, we have to at least add 0b10 to this (0x1 << 1)
+ * to account for the state bit. However, if the current seq is 7 (gp is 3 and
+ * state bit is 1), then it means the current grace period is already in
+ * progress so the next time the callback will run is at the end of grace
+ * period number gp+2. To account for the extra +1, we just overflow the LSB by
+ * adding another 0x1 and masking with ~0x1. In case no GP was in progress (RCU
+ * is idle), then the addition of the extra 0x1 and masking will have no
+ * effect. This is calculated as below.
+ */
 static inline unsigned long rcu_seq_snap(unsigned long *sp)
 {
 	unsigned long s;
-- 
2.17.0.441.gb46fe60e1d-goog

^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH v2] rcu: Add comment documenting how rcu_seq_snap works
  2018-05-12  2:20 [PATCH v2] rcu: Add comment documenting how rcu_seq_snap works Joel Fernandes (Google)
@ 2018-05-12  2:38 ` Randy Dunlap
  0 siblings, 0 replies; 2+ messages in thread
From: Randy Dunlap @ 2018-05-12  2:38 UTC (permalink / raw)
  To: Joel Fernandes (Google), linux-kernel; +Cc: paulmck

On 05/11/2018 07:20 PM, Joel Fernandes (Google) wrote:
> rcu_seq_snap may be tricky for someone looking at it for the first time.
> Lets document how it works with an example to make it easier.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> v2 changes: Corrections as suggested by Randy.
> 
>  kernel/rcu/rcu.h | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 003671825d62..533bc1087371 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -91,7 +91,29 @@ 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. */
> +/*
> + * Take a snapshot of the update side's sequence number.
> + *
> + * This function predicts what the grace period number will be the next
> + * time an RCU callback will be executed, given the current grace period's
> + * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is
> + * already in progress.
> + *
> + * We do this with a single addition and masking.
> + * For example, if RCU_SEQ_STATE_MASK=1 and the least significant bit (LSB) of
> + * the seq is used to track if a GP is in progress or not, its sufficient if we

                                                              it's

> + * add (2+1) and mask with ~1. Lets see why with an example:

                                  Let's

I.e., Let's not be so casual with (dropping) apostrophes.
But v3 can wait for other comments. :)

> + *
> + * Say the current seq is 6 which is 0b110 (gp is 3 and state bit is 0).
> + * To get the next GP number, we have to at least add 0b10 to this (0x1 << 1)
> + * to account for the state bit. However, if the current seq is 7 (gp is 3 and
> + * state bit is 1), then it means the current grace period is already in
> + * progress so the next time the callback will run is at the end of grace
> + * period number gp+2. To account for the extra +1, we just overflow the LSB by
> + * adding another 0x1 and masking with ~0x1. In case no GP was in progress (RCU
> + * is idle), then the addition of the extra 0x1 and masking will have no
> + * effect. This is calculated as below.
> + */
>  static inline unsigned long rcu_seq_snap(unsigned long *sp)
>  {
>  	unsigned long s;
> 


-- 
~Randy

^ permalink raw reply	[flat|nested] 2+ messages in thread

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