On Tue, May 7, 2019 at 2:14 AM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote:
>
> On Sun, May 05, 2019 at 09:16:53PM +0800, Junchang Wang wrote:
> > 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.
>
> There are a couple of ways to make this work. One of them is as shown
> in time_after() in the Linux kernel, and another is as shown in
> ULONG_CMP_GE().
>
Hi Paul,
Thanks for referencing me to these functions, which look great. I will
send a new patch soon.
Thanks,
--Junchang
> And yes, the C standard is less helpful than it might be in this area.
> You can thank old ones-complement machines and newer machines that
> trap on signed integer overflow. ;-)
>
> Thanx, Paul
>
> > Thanks,
> > --Junchang
> >
> > >> + WRITE_ONCE(*rrgp, *rrgp - 1);
> > >> goto retry;
> > >> }
> > >> }
> > >> --
> > >> 2.7.4
> > >>
> > >
> >
