Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion.

From: NeilBrown
Date: Mon Jul 30 2018 - 20:45:59 EST


On Fri, Jul 27 2018, Paul E. McKenney wrote:

> On Thu, Jul 26, 2018 at 08:18:15PM -0700, Paul E. McKenney wrote:
>> On Fri, Jul 27, 2018 at 11:04:37AM +1000, NeilBrown wrote:
>> > On Wed, Jul 25 2018, Paul E. McKenney wrote:
>> > >>
>> > >> Looks good ... except ... naming is hard.
>> > >>
>> > >> is_after_call_rcu_init() asserts where in the lifecycle we are,
>> > >> is_after_call_rcu() tests where in the lifecycle we are.
>> > >>
>> > >> The names are similar but the purpose is quite different.
>> > >> Maybe s/is_after_call_rcu_init/call_rcu_init/ ??
>> > >
>> > > How about rcu_head_init() and rcu_head_after_call_rcu()?
>>
>> Very well, I will pull this change in on my next rebase.
>
> Like this?

Hard to say - unwinding white-space damage in my head is too challenging
when newlines have been deleted :-(

Thanks,
NeilBrown

>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 45dfdcaee39bf6dd1785253c78e4805122ee8d93
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Date: Tue Jul 24 15:28:09 2018 -0700
>
> rcu: Provide functions for determining if call_rcu() has been invoked
>
> This commit adds rcu_head_init() and rcu_head_after_call_rcu()
> functions to help RCU users detect when another CPU has passed
> the specified rcu_head structure and function to call_rcu().
> The rcu_head_init() should be invoked before making the structure
> visible to RCU readers, and then the rcu_head_after_call_rcu()
> may be invoked from within an RCU read-side critical section on an
> rcu_head structure that was obtained during a traversal of the data
> structure in question. The rcu_head_after_call_rcu() function will
> return true if the rcu_head structure has already been passed (with
> the specified function) to call_rcu(), otherwise it will return false.
>
> If rcu_head_init() has not been invoked on the rcu_head structure
> or if the rcu_head (AKA callback) has already been invoked, then
> rcu_head_after_call_rcu() will do WARN_ON_ONCE().
>
> Reported-by: NeilBrown <neilb@xxxxxxxx> Signed-off-by: Paul
> E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> [ paulmck: Apply neilb
> feedback on naming. ]
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index
> e4f821165d0b..4db8bcacc51a 100644 --- a/include/linux/rcupdate.h +++
> b/include/linux/rcupdate.h @@ -857,6 +857,46 @@ static inline notrace
> void rcu_read_unlock_sched_notrace(void)
> #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
>
>
> +/* Has the specified rcu_head structure been handed to call_rcu()? */ +
> +/* + * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
> + * @rhp: The rcu_head structure to initialize. + * + * If you intend
> to invoke rcu_head_after_call_rcu() to test whether a + * given rcu_head
> structure has already been passed to call_rcu(), then + * you must also
> invoke this rcu_head_init() function on it just after + * allocating
> that structure. Calls to this function must not race with + * calls
> to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
> + */ +static inline void rcu_head_init(struct rcu_head *rhp) +{ +
> rhp->func = (rcu_callback_t)~0L; +} + +/* + * rcu_head_after_call_rcu -
> Has this rcu_head been passed to call_rcu()? + * @rhp: The rcu_head
> structure to test. + * @func: The function passed to call_rcu()
> along with @rhp. + * + * Returns @true if the @rhp has been passed to
> call_rcu() with @func, + * and @false otherwise. Emits a warning in any
> other case, including + * the case where @rhp has already been invoked
> after a grace period. + * Calls to this function must not race with
> callback invocation. One way + * to avoid such races is to enclose the
> call to rcu_head_after_call_rcu() + * in an RCU read-side critical section
> that includes a read-side fetch + * of the pointer to the structure
> containing @rhp. + */ +static inline bool +rcu_head_after_call_rcu(struct
> rcu_head *rhp, rcu_callback_t f) +{ + if (READ_ONCE(rhp->func) == f)
> + return true; + WARN_ON_ONCE(READ_ONCE(rhp->func) !=
> (rcu_callback_t)~0L); + return false; +} + +
> /* Transitional pre-consolidation compatibility definitions. */
>
> static inline void synchronize_rcu_bh(void)
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index
> 5dec94509a7e..4c56c1d98fb3 100644 --- a/kernel/rcu/rcu.h +++
> b/kernel/rcu/rcu.h @@ -224,6 +224,7 @@ void kfree(const void *);
> */
> static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head) {
> + rcu_callback_t f;
> unsigned long offset = (unsigned long)head->func;
>
> rcu_lock_acquire(&rcu_callback_map);
> @@ -234,7 +235,9 @@ static inline bool __rcu_reclaim(const char *rn,
> struct rcu_head *head)
> return true;
> } else {
> RCU_TRACE(trace_rcu_invoke_callback(rn, head);)
> - head->func(head); + f = head->func; +
> WRITE_ONCE(head->func, (rcu_callback_t)0L); + f(head);
> rcu_lock_release(&rcu_callback_map); return false;
> }

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