On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote: > Hi Paul, > > On 11/17/2020 6:10 AM, paulmck@xxxxxxxxxx wrote: > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxx/ > > Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> > > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ] > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > --- > > 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
