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    SubjectRe: [PATCH v3 1/4] rcu: Add comment documenting how rcu_seq_snap works
    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.

    ----------8<----------

    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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