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

[Jason Gunthorpe]

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>
> ---
>  mm/mmu_notifier.c | 27 ++++++++++++++++++---------
>  1 file changed, 18 insertions(+), 9 deletions(-)

Looks right to me

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

This was missed during testing because we don't run with
PROVE_RCU_LIST enabled

Jason