Re: blk->id read race: was: [PATCH v2 2/3] printk: add lockless buffer

Petr Mladek
2020-06-10T08:42:57Z

On Tue 2020-06-09 16:18:35, John Ogness wrote:
> On 2020-06-09, Petr Mladek <pmladek@suse.com> wrote:
> >> --- /dev/null
> >> +++ b/kernel/printk/printk_ringbuffer.c
> >> +/*
> >> + * Given a data ring (text or dict), put the associated descriptor of each
> >> + * data block from @lpos_begin until @lpos_end into the reusable state.
> >> + *
> >> + * If there is any problem making the associated descriptor reusable, either
> >> + * the descriptor has not yet been committed or another writer task has
> >> + * already pushed the tail lpos past the problematic data block. Regardless,
> >> + * on error the caller can re-load the tail lpos to determine the situation.
> >> + */
> >> +static bool data_make_reusable(struct printk_ringbuffer *rb,
> >> + struct prb_data_ring *data_ring,
> >> + unsigned long lpos_begin,
> >> + unsigned long lpos_end,
> >> + unsigned long *lpos_out)
> >> +{
> >> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> >> + struct prb_data_blk_lpos *blk_lpos;
> >> + struct prb_data_block *blk;
> >> + unsigned long tail_lpos;
> >> + enum desc_state d_state;
> >> + struct prb_desc desc;
> >> + unsigned long id;
> >> +
> >> + /*
> >> + * Using the provided @data_ring, point @blk_lpos to the correct
> >> + * blk_lpos within the local copy of the descriptor.
> >> + */
> >> + if (data_ring == &rb->text_data_ring)
> >> + blk_lpos = &desc.text_blk_lpos;
> >> + else
> >> + blk_lpos = &desc.dict_blk_lpos;
> >> +
> >> + /* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
> >> + while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
> >> + blk = to_block(data_ring, lpos_begin);
> >> + id = READ_ONCE(blk->id); /* LMM(data_make_reusable:A) */
> >
> > This would deserve some comment:
> >
> > 1. Compiler could not optimize out the read because there is a data
> > dependency on lpos_begin.
> >
> > 2. Compiler could not postpone the read because it is followed by
> > smp_rmb().
> >
> > So, is READ_ONCE() realy needed?
>
> I agree that it is not needed. Both the READ_ONCE() and its countering
> WRITE_ONCE() (data_alloc:B) only document the lockless shared access. I
> will remove both for the next version.

Sounds good.

> Do we still need a comment? Is it not obvious that there is a data
> dependency on @lpos_begin?

Sigh, I just wonder why I am always confusedby this. See below.

> blk = to_block(data_ring, lpos_begin);
> id = blk->id;
>
> > Well, blk->id clearly can be modified in parallel so we need to be
> > careful. There is smp_rmb() right below. Do we needed smp_rmb() also
> > before?
> >
> > What about the following scenario?:
> >
> >
> > CPU0 CPU1
> >
> > data_alloc()
> > data_push_tail()
> >
> > blk = to_block(data_ring, begin_lpos)
> > WRITE_ONCE(blk->id, id); /* LMM(data_alloc:B) */
> >
> > desc_push_tail()
> > data_push_tail()
> >
> > tail_lpos = data_ring->tail_lpos;
> > // see data_ring->tail_lpos already updated by CPU1
> >
> > data_make_reusable()
> >
> > // lpos_begin = tail_lpos via parameter
> > blk = to_block(data_ring, lpos_begin);
> > id = blk->id
> >
> > Now: CPU0 might see outdated blk->id before CPU1 wrote new value
> > because there is no read barrier betwen reading tail_lpos
> > and blk->id here.
>
> In your example, CPU1 is pushing the tail and then setting the block ID
> for the _newly_ allocated block, that is located is _before_ the new
> tail. If CPU0 sees the new tail already, it is still reading a valid
> block ID, which is _not_ from the block that CPU1 is in the process of
> writing.

Ah, I see. I wrongly assumed that both CPO0 and CPU1 are working with
the same block address. But if CPU0 sees the new tail_lpos, it is
already looking at another block. And it is the classic fight against
yet another potential CPUs that try to push the tail as well.

I wonder if the comment might look like:

/*
* No barrier is needed between reading tail_lpos and the related
* blk->id. Only CPU that modifies tail_lpos via cmpxchg is allowed
* to modify the related blk->id. CPUs that see the moved tail_lpos
* are looking at another block related to the new tail_lpos.
* It does not mater when the previous winner modifies the previous
* block.
*/

I am not sure how many people are confused like me. It is possible
that it is not worth it. I just know that I did this mistake
repeatedly ;-)

Best Regards,
Petr