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SubjectRe: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods
On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
> Hi Paul,
>
> On 11/17/2020 6:10 AM, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> >
> > There is a need for a polling interface for SRCU grace
> > periods, so this commit supplies get_state_synchronize_srcu(),
> > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> > purpose. The first can be used if future grace periods are inevitable
> > (perhaps due to a later call_srcu() invocation), the second if future
> > grace periods might not otherwise happen, and the third to check if a
> > grace period has elapsed since the corresponding call to either of the
> > first two.
> >
> > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> > the return value from either get_state_synchronize_srcu() or
> > start_poll_synchronize_srcu() must be passed in to a later call to
> > poll_state_synchronize_srcu().
> >
> > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> > Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
> > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> > include/linux/rcupdate.h | 2 ++
> > include/linux/srcu.h | 3 +++
> > include/linux/srcutiny.h | 1 +
> > kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> > 4 files changed, 56 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index de08264..e09c0d8 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -33,6 +33,8 @@
> > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
> > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
> > #define ulong2long(a) (*(long *)(&(a)))
> > +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
> > /* Exported common interfaces */
> > void call_rcu(struct rcu_head *head, rcu_callback_t func);
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index e432cc9..a0895bb 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> > int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> > void synchronize_srcu(struct srcu_struct *ssp);
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
> > #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index fed4a2d..e9bd6fb 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -16,6 +16,7 @@
> > struct srcu_struct {
> > short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
> > unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
> > + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
> > u8 srcu_gp_running; /* GP workqueue running? */
> > u8 srcu_gp_waiting; /* GP waiting for readers? */
> > struct swait_queue_head srcu_wq;
> > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > index 3bac1db..b405811 100644
> > --- a/kernel/rcu/srcutiny.c
> > +++ b/kernel/rcu/srcutiny.c
> > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> > ssp->srcu_gp_running = false;
> > ssp->srcu_gp_waiting = false;
> > ssp->srcu_idx = 0;
> > + ssp->srcu_idx_max = 0;
> > INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> > INIT_LIST_HEAD(&ssp->srcu_work.entry);
> > return 0;
> > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> > struct srcu_struct *ssp;
> > ssp = container_of(wp, struct srcu_struct, srcu_work);
> > - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> > + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > return; /* Already running or nothing to do. */
> > /* Remove recently arrived callbacks and wait for readers. */
> > @@ -147,14 +148,19 @@ void srcu_drive_gp(struct work_struct *wp)
> > * straighten that out.
> > */
> > WRITE_ONCE(ssp->srcu_gp_running, false);
> > - if (READ_ONCE(ssp->srcu_cb_head))
> > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>
> Should this be USHORT_CMP_LT ?

I believe that you are correct. As is, it works but does needless
grace periods.

> > schedule_work(&ssp->srcu_work);
> > }
> > EXPORT_SYMBOL_GPL(srcu_drive_gp);
> > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > {
> > + unsigned short cookie;
> > +
> > if (!READ_ONCE(ssp->srcu_gp_running)) {
> > + cookie = get_state_synchronize_srcu(ssp);
> > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
>
> I was thinking of a case which might break with this.
>
> Consider a scenario, where GP is in progress and kworker is right
> before below point, after executing callbacks:
>
> void srcu_drive_gp(struct work_struct *wp) {
> <snip>

We updated ->srcu_idx up here, correct? So it has bottom bit zero.

> while (lh) {
> <cb execution loop>
> }
> >>> CURRENT EXECUTION POINT

Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
due to an interrupt.

> WRITE_ONCE(ssp->srcu_gp_running, false);
>
> if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> schedule_work(&ssp->srcu_work);
> }
>
> Now, at this instance, srcu_gp_start_if_needed() runs and samples
> srcu_gp_running and returns, without updating srcu_idx_max
>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> if (!READ_ONCE(ssp->srcu_gp_running)) returns true
> <snip>
> }

This could happen in an interrupt handler, so with you thus far.

> kworker running srcu_drive_gp() resumes and returns without queueing a new
> schedule_work(&ssp->srcu_work); for new GP?
>
> Prior to this patch, call_srcu() enqueues a cb before entering
> srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
> queuing, and schedule a work for the new GP, for this scenario.
>
>
> WRITE_ONCE(ssp->srcu_gp_running, false);
> - if (READ_ONCE(ssp->srcu_cb_head))
> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> schedule_work(&ssp->srcu_work);
>
> So, should the "cookie" calculation and "srcu_idx_max" setting be moved
> outside of ssp->srcu_gp_running check and maybe return the same cookie to
> caller and use that as the returned cookie from
> start_poll_synchronize_srcu() ?
>
> srcu_gp_start_if_needed()
> cookie = get_state_synchronize_srcu(ssp);
> if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> WRITE_ONCE(ssp->srcu_idx_max, cookie);
> if (!READ_ONCE(ssp->srcu_gp_running)) {
> <snip>
> }

I believe that you are quite correct, thank you!

But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
in a timer handler. The race window is quite narrow, so testing it might
be a challenge...

This is what I end up with:

static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
unsigned short cookie;

cookie = get_state_synchronize_srcu(ssp);
if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
WRITE_ONCE(ssp->srcu_idx_max, cookie);
if (!READ_ONCE(ssp->srcu_gp_running)) {
if (likely(srcu_init_done))
schedule_work(&ssp->srcu_work);
else if (list_empty(&ssp->srcu_work.entry))
list_add(&ssp->srcu_work.entry, &srcu_boot_list);
}
}

Does that look plausible?

Thanx, Paul

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