* [PATCH] rcu: Add comment documenting how rcu_seq_snap works
@ 2018-05-12 0:33 Joel Fernandes (Google)
2018-05-12 1:56 ` Randy Dunlap
0 siblings, 1 reply; 2+ messages in thread
From: Joel Fernandes (Google) @ 2018-05-12 0:33 UTC (permalink / raw)
To: linux-kernel, 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>
---
kernel/rcu/rcu.h | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 003671825d62..004ace3d22c2 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -91,7 +91,28 @@ 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 0x110 (gp is 3 and state bit is 0).
+ * To get the next GP number, we have to atleast add 0x10 to this (0x1 << 1) to
+ * account for the state bit. However, if the current seq is 7 (GP num is 3
+ * and state bit is 1), then it means the current grace period is already
+ * in progress so the next the callback will run is at gp+2. To account for
+ * the extra +1, we just overflow the LSB by adding another 0x1 and masking
+ * with ~0x1. Incase no GP was in progress (RCU is idle), then the adding
+ * by 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 related [flat|nested] 2+ messages in thread
* [PATCH] rcu: Add comment documenting how rcu_seq_snap works
2018-05-12 0:33 [PATCH] rcu: Add comment documenting how rcu_seq_snap works Joel Fernandes (Google)
@ 2018-05-12 1:56 ` Randy Dunlap
0 siblings, 0 replies; 2+ messages in thread
From: Randy Dunlap @ 2018-05-12 1:56 UTC (permalink / raw)
To: Joel Fernandes (Google), linux-kernel, paulmck
On 05/11/2018 05:33 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>
> ---
> kernel/rcu/rcu.h | 23 ++++++++++++++++++++++-
> 1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 003671825d62..004ace3d22c2 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -91,7 +91,28 @@ 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 0x110 (gp is 3 and state bit is 0).
0b110
or 0x6
> + * To get the next GP number, we have to atleast add 0x10 to this (0x1 << 1) to
at least add 0b10
> + * account for the state bit. However, if the current seq is 7 (GP num is 3
> + * and state bit is 1), then it means the current grace period is already
> + * in progress so the next the callback will run is at gp+2. To account for
so the next time? the callback will run
> + * the extra +1, we just overflow the LSB by adding another 0x1 and masking
> + * with ~0x1. Incase no GP was in progress (RCU is idle), then the adding
In case
> + * by 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
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2018-05-12 0:33 [PATCH] rcu: Add comment documenting how rcu_seq_snap works Joel Fernandes (Google)
2018-05-12 1:56 ` Randy Dunlap
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