On Thu, May 02, 2019 at 07:59:51AM -0700, Paul E. McKenney wrote:
>On Tue, Apr 30, 2019 at 12:02:39PM +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.
>>
>> 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>
>
>First, thank you for doing this! Lots of good improvements!!!
>
>A few comments below.
>
> Thanx, Paul
>
>> ---
>> CodeSamples/defer/rcu_nest.c | 5 +++--
>> CodeSamples/defer/rcu_nest.h | 25 +++++++++++++++++--------
>> 2 files changed, 20 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..65ce203 100644
>> --- a/CodeSamples/defer/rcu_nest.h
>> +++ b/CodeSamples/defer/rcu_nest.h
>> @@ -24,8 +24,9 @@ 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 +40,8 @@ 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 +53,21 @@ 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
>
>s/writting/writing/
>
>> + * time period, the grace-period counter might have advanced very far.
>> + * In this case, we force the reader to start over.
>> + */
>> +
>> if (((tmp & RCU_GP_CTR_NEST_MASK) == 1) &&
>> - ((rcu_gp_ctr - tmp) > (RCU_GP_CTR_NEST_MASK << 8)) != 0) {
>> - (*rrgp)--;
>> + (READ_ONCE(rcu_gp_ctr) > tmp + MAX_GP_ADV_DISTANCE)) {
>
>Does this work correctly if the value of tmp is just a bit less than
>ULONG_MAX? It looks to me like it does not.
>
Hi Paul,
Thanks for reviewing the code. Yes, the statement you pointed out is buggy
when both rcu_gp_ctr and tmp are close to ULONG_MAX. I was trying to
rewrite to code to check for true overflow, but, as you have pointed out
in a previous mail, that could make the code complex if we want to be
exact. So I go back to the logic you used before. Please check the updated
code below:
+
+ /*
+ * 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.
+ */
+
if (((tmp & RCU_GP_CTR_NEST_MASK) == 1) &&
- ((rcu_gp_ctr - tmp) > (RCU_GP_CTR_NEST_MASK << 8)) != 0) {
- (*rrgp)--;
+ (READ_ONCE(rcu_gp_ctr) - (tmp-1)) > MAX_GP_ADV_DISTANCE) {
+ WRITE_ONCE(*rrgp, *rrgp - 1);
goto retry;
}
}
Note that if *rcu_gp_ctr* has not been changed since its value was first
read and stored in *tmp*, then its value is less than *tmp* by 1.
Since both *rcu_gp_ctr* and *tmp* are now unsigned long integers,
(rcu_gp_ctr - tmp) will generate a very large integer number, so I replace
that with (READ_ONCE(rcu_gp_ctr) - (tmp-1)). Please let me know if that looks OK.
Thanks,
--Junchang
>> + WRITE_ONCE(*rrgp, *rrgp - 1);
>> goto retry;
>> }
>> }
>> --
>> 2.7.4
>>
>
