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

Neeraj Upadhyay
2020-11-25T04:40:02Z

On 11/25/2020 1:00 AM, Paul E. McKenney wrote:
> On Tue, Nov 24, 2020 at 10:44:24AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> 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!
>
> Are you willing to give an ack or reviewed-by for either patch?
>
> Thanx, Paul
>

For the version in rcu -dev

Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>

Thanks
Neeraj

>> 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

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation