Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

Neeraj Upadhyay
2020-11-24T05:14:40Z

On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
> On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> 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);
>
> Good point, added and under test.
>
>> 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?
>
> How about like this?

Looks good!

Thanks
Neeraj

>
> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> {
> unsigned short cookie;
>
> cookie = get_state_synchronize_srcu(ssp);
> if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
> return;
> 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);
> }
> }
>
> Testing this next. ;-)
>
> Thanx, Paul
>
>> 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)
>>> {
>>>
>>
>> --
>> 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