Re: [PATCH v5.2 1/2] locking/rwsem: Add missing ACQUIRE to read_slowpath exit when queue is empty

[Jan Stancek]

----- Original Message -----
> From: Jan Stancek <jstancek@xxxxxxxxxx>
> 
> [ Upstream commit e1b98fa316648420d0434d9ff5b92ad6609ba6c3 ]
> 
> LTP mtest06 has been observed to occasionally hit "still mapped when
> deleted" and following BUG_ON on arm64.
> 
> 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 the pagefault handler (under mmap_sem read lock) calls
> find_vma(), vmacache_valid() wrongly reports vmacache as valid.
> 
> After rwsem down_read() returns via 'queue empty' path (as of v5.2),
> it does so without an 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;
> 
> The problem can be reproduced by running LTP mtest06 in a loop and
> building the kernel (-j $NCPUS) in parallel. It does reproduces since
> v4.20 on arm64 HPE Apollo 70 (224 CPUs, 256GB RAM, 2 nodes). It
> triggers reliably in about an hour.
> 
> The patched kernel ran fine for 10+ hours.
> 
> Signed-off-by: Jan Stancek <jstancek@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Reviewed-by: Will Deacon <will@xxxxxxxxxx>
> Acked-by: Waiman Long <longman@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: dbueso@xxxxxxx
> Fixes: 4b486b535c33 ("locking/rwsem: Exit read lock slowpath if queue empty &
> no writer")
> Link:
> https://lkml.kernel.org/r/50b8914e20d1d62bb2dee42d342836c2c16ebee7.1563438048.git.jstancek@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
> 
> This is a backport for the v5.2 stable tree. There were multiple reports
> of this issue being hit.
> 
> Given that there were a few changes to the code around this, I'd
> appreciate an ack before pulling it in.

ACK, both look good to me.
I also re-ran reproducer with this series applied on top of 5.2.10, it PASS-ed.

Thanks,
Jan