On Wed, May 08, 2019 at 10:26:01PM +0800, Junchang Wang wrote:
> Variable "rcu_gp_ctr" is incremented by the updater and is fetched by
> readers concurrently. So protect this variable by using READ_ONCE()
> and WRITE_ONCE().
>
> Per-thread variable "rcu_read_gp" is updated by the reader and is read
> by the updater. So protect it by using READ_ONCE() and WRITE_ONCE().
>
> The type of "rcu_gp_ctr" is changed to unsigned long because the behavior
> of the overflow of a signed long integer is not well defined in C yet.
> "rcu_gp_ctr" could wrap around. To compare it to "rcu_reader_gp" correctly,
> helper function ULONG_CMP_GE() is used.
>
> Refine the code snippet in "rcu_read_lock" that allows a reader to start
> over. In this version, we add micro MAX_GP_ADV_DISTANCE which is by
> default set to (RCU_GP_CTR_NEST_MASK << 8). Once a reader notices that
> MAX_GP_ADV_DISTANCE grace-periods have elapsed since fetching the value of
> "rcu_reader_gp", the reader starts over.
>
> Signed-off-by: Junchang Wang <junchangwang@xxxxxxxxx>
Queued and pushed, thank you!!!
Thanx, Paul
> ---
> Changes from v1:
> - Use ULONG_CMP_GE() to compare variables correctly.
>
>
> CodeSamples/defer/rcu_nest.c | 5 +++--
> CodeSamples/defer/rcu_nest.h | 32 ++++++++++++++++++++++++--------
> 2 files changed, 27 insertions(+), 10 deletions(-)
>
> diff --git a/CodeSamples/defer/rcu_nest.c b/CodeSamples/defer/rcu_nest.c
> index 64e4087..362f466 100644
> --- a/CodeSamples/defer/rcu_nest.c
> +++ b/CodeSamples/defer/rcu_nest.c
> @@ -35,7 +35,7 @@ void synchronize_rcu(void)
>
> /* Advance to a new grace-period number, enforce ordering. */
>
> - rcu_gp_ctr += RCU_GP_CTR_BOTTOM_BIT;
> + WRITE_ONCE(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR_BOTTOM_BIT);
> smp_mb();
>
> /*
> @@ -45,7 +45,8 @@ void synchronize_rcu(void)
>
> for_each_thread(t) {
> while (rcu_gp_ongoing(t) &&
> - ((per_thread(rcu_reader_gp, t) - rcu_gp_ctr) < 0)) {
> + ((READ_ONCE(per_thread(rcu_reader_gp, t)) -
> + rcu_gp_ctr) < 0)) {
> /*@@@ poll(NULL, 0, 10); */
> barrier();
> }
> diff --git a/CodeSamples/defer/rcu_nest.h b/CodeSamples/defer/rcu_nest.h
> index bcc4cde..7e7b7de 100644
> --- a/CodeSamples/defer/rcu_nest.h
> +++ b/CodeSamples/defer/rcu_nest.h
> @@ -20,12 +20,17 @@
>
> #include "rcu_pointer.h"
>
> +/* Borrow from rcupdate.h in Linux kernel */
> +#define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> +#define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> +
> DEFINE_SPINLOCK(rcu_gp_lock);
> #define RCU_GP_CTR_SHIFT 7
> #define RCU_GP_CTR_BOTTOM_BIT (1 << RCU_GP_CTR_SHIFT)
> #define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BOTTOM_BIT - 1)
> -long rcu_gp_ctr = 0; /* increment by RCU_GP_CTR_BOTTOM_BIT each gp. */
> -DEFINE_PER_THREAD(long, rcu_reader_gp);
> +#define MAX_GP_ADV_DISTANCE (RCU_GP_CTR_NEST_MASK << 8)
> +unsigned long rcu_gp_ctr = 0; /* increment by RCU_GP_CTR_BOTTOM_BIT each gp. */
> +DEFINE_PER_THREAD(unsigned long, rcu_reader_gp);
>
> static inline int rcu_gp_ongoing(int cpu)
> {
> @@ -39,8 +44,8 @@ static void rcu_init(void)
>
> static void rcu_read_lock(void)
> {
> - long tmp;
> - long *rrgp;
> + unsigned long tmp;
> + unsigned long *rrgp;
>
> /*
> * If this is the outermost RCU read-side critical section,
> @@ -52,13 +57,24 @@ static void rcu_read_lock(void)
> retry:
> tmp = *rrgp;
> if ((tmp & RCU_GP_CTR_NEST_MASK) == 0)
> - tmp = rcu_gp_ctr;
> + tmp = READ_ONCE(rcu_gp_ctr);
> tmp++;
> - *rrgp = tmp;
> + WRITE_ONCE(*rrgp, tmp);
> smp_mb();
> +
> + /*
> + * A reader could be suspended in between fetching the value of *rrgp
> + * and writting the updated value back into *rrgp. During this
> + * time period, the grace-period counter might have advanced very far.
> + * In this case, we force the reader to start over. One special case
> + * is that "rcu_gp_ctr" may have wrapped around while "tmp" is close to
> + * ULONG_MAX. To handle this correctly, we adopt the helper function
> + * ULONG_CMP_GE.
> + */
> +
> if (((tmp & RCU_GP_CTR_NEST_MASK) == 1) &&
> - ((rcu_gp_ctr - tmp) > (RCU_GP_CTR_NEST_MASK << 8)) != 0) {
> - (*rrgp)--;
> + ULONG_CMP_GE(READ_ONCE(rcu_gp_ctr), tmp + MAX_GP_ADV_DISTANCE)) {
> + WRITE_ONCE(*rrgp, *rrgp - 1);
> goto retry;
> }
> }
> --
> 2.7.4
>
