lkml.org 
[lkml]   [2018]   [May]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH v3 1/4] rcu: Add comment documenting how rcu_seq_snap works
    Date
    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
    + * 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
    + * 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
    + * overflow didn't occur. This masking is needed because incase 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
    \
     
     \ /
      Last update: 2018-05-21 06:43    [W:4.726 / U:0.184 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site