On Tue, Jul 24 2018, Paul E. McKenney wrote: > On Tue, Jul 24, 2018 at 07:52:03AM +1000, NeilBrown wrote: >> On Mon, Jul 23 2018, Paul E. McKenney wrote: >> >> > On Mon, Jul 23, 2018 at 09:13:43AM +1000, NeilBrown wrote: >> >> On Sun, Jul 22 2018, Paul E. McKenney wrote: >> >> > >> >> > One issue is that the ->func pointer can legitimately be NULL while on >> >> > RCU's callback lists. This happens when someone invokes kfree_rcu() >> >> > with the rcu_head structure at the beginning of the enclosing structure. >> >> > I could add an offset to avoid this, or perhaps the kmalloc() folks >> >> > could be persuaded Rao Shoaib's patch moving kfree_rcu() handling to >> >> > the slab allocators, so that RCU only ever sees function pointers in >> >> > the ->func field. >> >> > >> >> > Either way, this should be hidden behind an API to allow adjustments >> >> > to be made if needed. Maybe something like is_after_call_rcu()? >> >> > This would (for example) allow debug-object checks to be used to catch >> >> > check-after-free bugs. >> >> > >> >> > Would something of that sort work for you? >> >> >> >> Yes, if you could provide an is_after_call_rcu() API, that would >> >> perfectly suit my use-case. >> > >> > After beating my head against the object-debug code a bit, I have to ask >> > if it would be OK for you if the is_after_call_rcu() API also takes the >> > function that was passed to RCU. >> >> Sure. It feels a bit clumsy, but I can see it could be easier to make >> robust. >> So yes: I'm fine with pass the same function and rcu_head to both >> call_rcu() and is_after_call_rcu(). Actually, when I say it like that, >> it seems less clumsy :-) > > How about like this? (It needs refinements, like lockdep, but should > get the gist.) >
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/ ??
Thanks,
NeilBrown
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 5aa0ebf4799b8bddbbd0124db1c008526e99fc7c
> Author: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Date: Tue Jul 24 15:28:09 2018 -0700
>
> rcu: Provide functions for determining if call_rcu() has been invoked
>
> This commit adds is_after_call_rcu() and is_after_call_rcu_init()
> functions to help RCU users detect when another CPU has passed
> the specified rcu_head structure and function to call_rcu().
> The is_after_call_rcu_init() should be invoked before making the
> structure visible to RCU readers, and then the is_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 is_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 is_after_call_rcu_init() has not been invoked on the rcu_head
> structure or if the rcu_head (AKA callback) has already been invoked,
> then is_after_call_rcu() will do WARN_ON_ONCE().
>
> Reported-by: NeilBrown <ne...@suse.com>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index e4f821165d0b..82e5a91539b5 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -857,6 +857,45 @@ 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()? */
> +
> +/*
> + * is_after_call_rcu_init - Initialize rcu_head for is_after_call_rcu()
> + * @rhp: The rcu_head structure to initialize.
> + *
> + * If you intend to invoke is_after_call_rcu() to test whether a given
> + * rcu_head structure has already been passed to call_rcu(), then you must
> + * also invoke this is_after_call_rcu_init() function on it just after
> + * allocating that structure. Calls to this function must not race with
> + * calls to call_rcu(), is_after_call_rcu(), or callback invocation.
> + */
> +static inline void is_after_call_rcu_init(struct rcu_head *rhp)
> +{
> + rhp->func = (rcu_callback_t)~0L;
> +}
> +
> +/*
> + * is_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 is_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 is_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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