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

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

[Sasha Levin]

From: Jan Stancek <jstancek@redhat.com>

[ 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@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Will Deacon <will@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dbueso@suse.de
Fixes: 4b486b535c33 ("locking/rwsem: Exit read lock slowpath if queue empty & no writer")
Link: https://lkml.kernel.org/r/50b8914e20d1d62bb2dee42d342836c2c16ebee7.1563438048.git.jstancek@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

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.

 kernel/locking/rwsem-xadd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 0b1f779572402..397dedc58432d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -454,6 +454,8 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
 		 * been set in the count.
 		 */
 		if (atomic_long_read(&sem->count) >= 0) {
+			/* Provide lock ACQUIRE */
+			smp_acquire__after_ctrl_dep();
 			raw_spin_unlock_irq(&sem->wait_lock);
 			rwsem_set_reader_owned(sem);
 			lockevent_inc(rwsem_rlock_fast);
-- 
2.20.1

[PATCH v5.2 2/2] lcoking/rwsem: Add missing ACQUIRE to read_slowpath sleep loop

[Sasha Levin]

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 99143f82a255e7f054bead8443462fae76dd829e ]

While reviewing another read_slowpath patch, both Will and I noticed
another missing ACQUIRE, namely:

  X = 0;

  CPU0			CPU1

  rwsem_down_read()
    for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);

                        X = 1;
                        rwsem_up_write();
                          rwsem_mark_wake()
                            atomic_long_add(adjustment, &sem->count);
                            smp_store_release(&waiter->task, NULL);

      if (!waiter.task)
        break;

      ...
    }

  r = X;

Allows 'r == 0'.

Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

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.

 kernel/locking/rwsem-xadd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 397dedc58432d..385ebcfc31a6d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -485,8 +485,10 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
 	/* wait to be given the lock */
 	while (true) {
 		set_current_state(state);
-		if (!waiter.task)
+		if (!smp_load_acquire(&waiter.task)) {
+			/* Orders against rwsem_mark_wake()'s smp_store_release() */
 			break;
+		}
 		if (signal_pending_state(state, current)) {
 			raw_spin_lock_irq(&sem->wait_lock);
 			if (waiter.task)
-- 
2.20.1

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@redhat.com>
> 
> [ 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@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Will Deacon <will@kernel.org>
> Acked-by: Waiman Long <longman@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: dbueso@suse.de
> Fixes: 4b486b535c33 ("locking/rwsem: Exit read lock slowpath if queue empty &
> no writer")
> Link:
> https://lkml.kernel.org/r/50b8914e20d1d62bb2dee42d342836c2c16ebee7.1563438048.git.jstancek@redhat.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> 
> 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

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

[Sasha Levin]

On Tue, Aug 27, 2019 at 10:11:39AM -0400, Jan Stancek wrote:
>ACK, both look good to me.
>I also re-ran reproducer with this series applied on top of 5.2.10, it PASS-ed.

I've queued both for 5.2, thanks Jan.

--
Thanks,
Sasha