Re: [PATCH] mm/mmu_notifier: silence PROVE_RCU_LIST warnings

[Paul E. McKenney]

On Tue, Mar 17, 2020 at 01:56:40PM -0400, Qian Cai wrote:
> It is safe to traverse mm->notifier_subscriptions->list either under
> SRCU read lock or mm->notifier_subscriptions->lock using
> hlist_for_each_entry_rcu(). Silence the PROVE_RCU_LIST false positives,
> for example,
> 
>  WARNING: suspicious RCU usage
>  -----------------------------
>  mm/mmu_notifier.c:484 RCU-list traversed in non-reader section!!
> 
>  other info that might help us debug this:
> 
>  rcu_scheduler_active = 2, debug_locks = 1
>  3 locks held by libvirtd/802:
>   #0: ffff9321e3f58148 (&mm->mmap_sem#2){++++}, at: do_mprotect_pkey+0xe1/0x3e0
>   #1: ffffffff91ae6160 (mmu_notifier_invalidate_range_start){+.+.}, at: change_p4d_range+0x5fa/0x800
>   #2: ffffffff91ae6e08 (srcu){....}, at: __mmu_notifier_invalidate_range_start+0x178/0x460
> 
>  stack backtrace:
>  CPU: 7 PID: 802 Comm: libvirtd Tainted: G          I       5.6.0-rc6-next-20200317+ #2
>  Hardware name: HP ProLiant BL460c Gen8, BIOS I31 11/02/2014
>  Call Trace:
>   dump_stack+0xa4/0xfe
>   lockdep_rcu_suspicious+0xeb/0xf5
>   __mmu_notifier_invalidate_range_start+0x3ff/0x460
>   change_p4d_range+0x746/0x800
>   change_protection+0x1df/0x300
>   mprotect_fixup+0x245/0x3e0
>   do_mprotect_pkey+0x23b/0x3e0
>   __x64_sys_mprotect+0x51/0x70
>   do_syscall_64+0x91/0xae8
>   entry_SYSCALL_64_after_hwframe+0x49/0xb3
> 
> Signed-off-by: Qian Cai <cai@xxxxxx>

>From an RCU perspective:

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  mm/mmu_notifier.c | 27 ++++++++++++++++++---------
>  1 file changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index ef3973a5d34a..06852b896fa6 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -307,7 +307,8 @@ static void mn_hlist_release(struct mmu_notifier_subscriptions *subscriptions,
>  	 * ->release returns.
>  	 */
>  	id = srcu_read_lock(&srcu);
> -	hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist)
> +	hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu))
>  		/*
>  		 * If ->release runs before mmu_notifier_unregister it must be
>  		 * handled, as it's the only way for the driver to flush all
> @@ -370,7 +371,8 @@ int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
>  
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(subscription,
> -				 &mm->notifier_subscriptions->list, hlist) {
> +				 &mm->notifier_subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		if (subscription->ops->clear_flush_young)
>  			young |= subscription->ops->clear_flush_young(
>  				subscription, mm, start, end);
> @@ -389,7 +391,8 @@ int __mmu_notifier_clear_young(struct mm_struct *mm,
>  
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(subscription,
> -				 &mm->notifier_subscriptions->list, hlist) {
> +				 &mm->notifier_subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		if (subscription->ops->clear_young)
>  			young |= subscription->ops->clear_young(subscription,
>  								mm, start, end);
> @@ -407,7 +410,8 @@ int __mmu_notifier_test_young(struct mm_struct *mm,
>  
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(subscription,
> -				 &mm->notifier_subscriptions->list, hlist) {
> +				 &mm->notifier_subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		if (subscription->ops->test_young) {
>  			young = subscription->ops->test_young(subscription, mm,
>  							      address);
> @@ -428,7 +432,8 @@ void __mmu_notifier_change_pte(struct mm_struct *mm, unsigned long address,
>  
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(subscription,
> -				 &mm->notifier_subscriptions->list, hlist) {
> +				 &mm->notifier_subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		if (subscription->ops->change_pte)
>  			subscription->ops->change_pte(subscription, mm, address,
>  						      pte);
> @@ -476,7 +481,8 @@ static int mn_hlist_invalidate_range_start(
>  	int id;
>  
>  	id = srcu_read_lock(&srcu);
> -	hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist) {
> +	hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		const struct mmu_notifier_ops *ops = subscription->ops;
>  
>  		if (ops->invalidate_range_start) {
> @@ -528,7 +534,8 @@ mn_hlist_invalidate_end(struct mmu_notifier_subscriptions *subscriptions,
>  	int id;
>  
>  	id = srcu_read_lock(&srcu);
> -	hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist) {
> +	hlist_for_each_entry_rcu(subscription, &subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		/*
>  		 * Call invalidate_range here too to avoid the need for the
>  		 * subsystem of having to register an invalidate_range_end
> @@ -582,7 +589,8 @@ void __mmu_notifier_invalidate_range(struct mm_struct *mm,
>  
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(subscription,
> -				 &mm->notifier_subscriptions->list, hlist) {
> +				 &mm->notifier_subscriptions->list, hlist,
> +				 srcu_read_lock_held(&srcu)) {
>  		if (subscription->ops->invalidate_range)
>  			subscription->ops->invalidate_range(subscription, mm,
>  							    start, end);
> @@ -714,7 +722,8 @@ find_get_mmu_notifier(struct mm_struct *mm, const struct mmu_notifier_ops *ops)
>  
>  	spin_lock(&mm->notifier_subscriptions->lock);
>  	hlist_for_each_entry_rcu(subscription,
> -				 &mm->notifier_subscriptions->list, hlist) {
> +				 &mm->notifier_subscriptions->list, hlist,
> +				 lockdep_is_held(&mm->notifier_subscriptions->lock)) {
>  		if (subscription->ops != ops)
>  			continue;
>  
> -- 
> 2.21.0 (Apple Git-122.2)
>