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Date: Sun, 22 Nov 2020 19:57:26 +0530
From: Neeraj Upadhyay <neeraju@...eaurora.org>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for
Tiny SRCU grace periods
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@...nel.org wrote:
>>> From: "Paul E. McKenney" <paulmck@...nel.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@...il.com>
>>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.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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