Messages in this thread |  | | Subject | Re: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods | From | Neeraj Upadhyay <> | Date | Sun, 22 Nov 2020 19:57:26 +0530 |
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On 11/21/2020 5:43 AM, Paul E. McKenney wrote: > 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. > Looking more, issue can happen, even when kworker is waiting for GP completion @
swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
Other process can call call_srcu() and skip srcu_idx_max update, as ssp->srcu_gp_running is true.
Thanks Neeraj
>> 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?
Looks good.
> > Thanx, Paul >
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