Messages in this thread |  | | Subject | Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods | From | Neeraj Upadhyay <> | Date | Mon, 23 Nov 2020 10:13:13 +0530 |
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On 11/21/2020 6:29 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. ] > [ paulmck: Apply feedback from Neeraj Upadhyay. ] > Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/ > 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 d9edb67..c7f0c1f 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..b073175 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;
Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max); WARN_ON(ssp->srcu_idx & 1);
Thanks Neeraj
> @@ -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. */ > @@ -147,13 +148,18 @@ 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))) > 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; > + > + cookie = get_state_synchronize_srcu(ssp); > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie)) > + WRITE_ONCE(ssp->srcu_idx_max, cookie);
Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid scheduling work?
Thanks Neeraj
> if (!READ_ONCE(ssp->srcu_gp_running)) { > if (likely(srcu_init_done)) > schedule_work(&ssp->srcu_work); > @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp) > } > EXPORT_SYMBOL_GPL(synchronize_srcu); > > +/* > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie > + */ > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp) > +{ > + unsigned long ret; > + > + barrier(); > + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1; > + barrier(); > + return ret & USHRT_MAX; > +} > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu); > + > +/* > + * start_poll_synchronize_srcu - Provide cookie and start grace period > + * > + * The difference between this and get_state_synchronize_srcu() is that > + * this function ensures that the poll_state_synchronize_srcu() will > + * eventually return the value true. > + */ > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp) > +{ > + unsigned long ret = get_state_synchronize_srcu(ssp); > + > + srcu_gp_start_if_needed(ssp); > + return ret; > +} > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu); > + > +/* > + * poll_state_synchronize_srcu - Has cookie's grace period ended? > + */ > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie) > +{ > + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie); > + > + barrier(); > + return ret; > +} > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu); > + > /* Lockdep diagnostics. */ > void __init rcu_scheduler_starting(void) > { >
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