Re: [PATCH] locking/rwsem: use read_acquire in read_slowpath exit when queue is empty

[Jan Stancek]

----- Original Message -----
> On 7/16/19 12:04 PM, Jan Stancek wrote:
> > LTP mtest06 has been observed to rarely hit "still mapped when deleted"
> > and following BUG_ON on arm64:
> >   page:ffff7e02fa37e480 refcount:3 mapcount:1 mapping:ffff80be3d678ab0
> >   index:0x0
> >   xfs_address_space_operations [xfs]
> >   flags: 0xbfffe000000037(locked|referenced|uptodate|lru|active)
> >   page dumped because: VM_BUG_ON_PAGE(page_mapped(page))
> >   ------------[ cut here ]------------
> >   kernel BUG at mm/filemap.c:171!
> >   Internal error: Oops - BUG: 0 [#1] SMP
> >   CPU: 220 PID: 154292 Comm: mmap1 Not tainted 5.2.0-0ecfebd.cki #1
> >   Hardware name: HPE Apollo 70 /C01_APACHE_MB , BIOS L50_5.13_1.10
> >   05/17/2019
> >   pstate: 40400089 (nZcv daIf +PAN -UAO)
> >   pc : unaccount_page_cache_page+0x17c/0x1a0
> >   lr : unaccount_page_cache_page+0x17c/0x1a0
> >   Call trace:
> >   unaccount_page_cache_page+0x17c/0x1a0
> >   delete_from_page_cache_batch+0xa0/0x300
> >   truncate_inode_pages_range+0x1b8/0x640
> >   truncate_inode_pages_final+0x88/0xa8
> >   evict+0x1a0/0x1d8
> >   iput+0x150/0x240
> >   dentry_unlink_inode+0x120/0x130
> >   __dentry_kill+0xd8/0x1d0
> >   dentry_kill+0x88/0x248
> >   dput+0x168/0x1b8
> >   __fput+0xe8/0x208
> >   ____fput+0x20/0x30
> >   task_work_run+0xc0/0xf0
> >   do_notify_resume+0x2b0/0x328
> >   work_pending+0x8/0x10
> >
> > The extra mapcount originated from pagefault handler, which handled
> > pagefault for vma that has already been detached. vma is detached
> > under mmap_sem write lock by detach_vmas_to_be_unmapped(), which
> > also invalidates vmacache.
> >
> > When pagefault handler (under mmap_sem read lock) called find_vma(),
> > vmacache_valid() wrongly reported vmacache as valid.
> >
> > After rwsem down_read() returns via 'queue empty' path (as of v5.2),
> > it does so without issuing read_acquire on sem->count:
> >   down_read
> >     __down_read
> >       rwsem_down_read_failed
> >         __rwsem_down_read_failed_common
> >           raw_spin_lock_irq(&sem->wait_lock);
> >           if (list_empty(&sem->wait_list)) {
> >             if (atomic_long_read(&sem->count) >= 0) {
> >               raw_spin_unlock_irq(&sem->wait_lock);
> >               return sem;
> >
> > Suspected problem here is that last *_acquire on down_read() side
> > happens before write side issues *_release:
> >   1. writer: has the lock
> >   2. reader: down_read() issues *read_acquire on entry
> >   3. writer: mm->vmacache_seqnum++; downgrades lock (*fetch_add_release)
> >   4. reader: __rwsem_down_read_failed_common() finds it can take lock and
> >   returns
> >   5. reader: observes stale mm->vmacache_seqnum
> >
> > I can reproduce the problem by running LTP mtest06 in a loop and building
> > kernel (-j $NCPUS) in parallel. It does reproduce since v4.20 up to v5.2
> > on arm64 HPE Apollo 70 (224 CPUs, 256GB RAM, 2 nodes). It triggers reliably
> > within ~hour. Patched kernel ran fine for 5+ hours with clean dmesg.
> > Tests were done against v5.2, since commit cf69482d62d9 ("locking/rwsem:
> > Enable readers spinning on writer") makes it much harder to reproduce.
> The explanation makes sense to me. Thanks for the investigation.
> >
> > Related:
> > https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/mtest06/mmap1.c
> > Related: commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in
> > munmap")
> > Fixes: 4b486b535c33 ("locking/rwsem: Exit read lock slowpath if queue empty
> > & no writer")
> >
> > Signed-off-by: Jan Stancek <jstancek@xxxxxxxxxx>
> > Cc: Waiman Long <longman@xxxxxxxxxx>
> > Cc: Davidlohr Bueso <dbueso@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > ---
> >  kernel/locking/rwsem.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> > index 37524a47f002..757b198d7a5b 100644
> > --- a/kernel/locking/rwsem.c
> > +++ b/kernel/locking/rwsem.c
> > @@ -1030,7 +1030,7 @@ static inline bool rwsem_reader_phase_trylock(struct
> > rw_semaphore *sem,
> >  		 * exit the slowpath and return immediately as its
> >  		 * RWSEM_READER_BIAS has already been set in the count.
> >  		 */
> > -		if (adjustment && !(atomic_long_read(&sem->count) &
> > +		if (adjustment && !(atomic_long_read_acquire(&sem->count) &
> >  		     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> >  			raw_spin_unlock_irq(&sem->wait_lock);
> >  			rwsem_set_reader_owned(sem);
> 
> The chance of taking this path is not that high. So instead of
> increasing the cost of the test by adding an acquire barrier,

Yes, that is good point.

> how about
> just adding smp_mb__after_spinlock() before spin_unlock_irq(). This
> should have the same effect of making sure that no stale data will be
> used in the read-lock critical section.

I'll redo the tests with smp_mb__after_spinlock().

Thanks,
Jan

> 
> -Longman
> 
>