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 | Mon, 23 Nov 2020 10:04:23 +0530 |
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On 11/22/2020 11:31 PM, Paul E. McKenney wrote: > On Sun, Nov 22, 2020 at 07:57:26PM +0530, Neeraj Upadhyay wrote: >> 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. > > Good point! Does this mean that additional changes are required, > or does the fix below cover this situation as well? > > Thanx, Paul >
I think the current fix covers this. Just wanted to higlight that the window is not small and a rcutorture test case might be able to uncover the issue?
Thanks Neeraj
>> 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 >>> >> >> -- >> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of >> the Code Aurora Forum, hosted by The Linux Foundation
-- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation
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