Re: [PATCH v2 2/3] printk: add lockless buffer

John Ogness
2020-05-18T13:04:10Z

Hi,

For the v2 I was supposed to have fixed a bug in v1. The cover letter
stated:

- data_push_tail(): continue loop rather than try cmpxchg() if
tail LPOS was pushed

Unfortunately, the fix was incorrect and actually made it worse. The
explanation and patch are below.

On 2020-05-01, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> new file mode 100644
> index 000000000000..e0a66468d4f3
> --- /dev/null
> +++ b/kernel/printk/printk_ringbuffer.c
...
> +/*
> + * Advance the data ring tail to at least @lpos. This function puts
> + * descriptors into the reusable state if the tail is pushed beyond
> + * their associated data block.
> + */
> +static bool data_push_tail(struct printk_ringbuffer *rb,
> + struct prb_data_ring *data_ring,
> + unsigned long lpos)
> +{
> + unsigned long tail_lpos;
> + unsigned long next_lpos;
> +
> + /* If @lpos is not valid, there is nothing to do. */
> + if (lpos == INVALID_LPOS)
> + return true;
> +
> + tail_lpos = atomic_long_read(&data_ring->tail_lpos);
> +
> + do {
> + /* Done, if the tail lpos is already at or beyond @lpos. */
> + if ((lpos - tail_lpos) - 1 >= DATA_SIZE(data_ring))
> + break;
> +
> + /*
> + * Make all descriptors reusable that are associated with
> + * data blocks before @lpos.
> + */
> + if (!data_make_reusable(rb, data_ring, tail_lpos, lpos,
> + &next_lpos)) {
> + /*
> + * Guarantee the descriptor state loaded in
> + * data_make_reusable() is performed before reloading
> + * the tail lpos. The failed data_make_reusable() may
> + * be due to a newly recycled descriptor causing
> + * the tail lpos to have been previously pushed. This
> + * pairs with desc_reserve:D.
> + *
> + * Memory barrier involvement:
> + *
> + * If data_make_reusable:D reads from desc_reserve:G,
> + * then data_push_tail:B reads from data_push_tail:D.
> + *
> + * Relies on:
> + *
> + * MB from data_push_tail:D to desc_reserve:G
> + * matching
> + * RMB from data_make_reusable:D to data_push_tail:B
> + *
> + * Note: data_push_tail:D and desc_reserve:G can be
> + * different CPUs. However, the desc_reserve:G
> + * CPU (which performs the full memory barrier)
> + * must have previously seen data_push_tail:D.
> + */
> + smp_rmb(); /* LMM(data_push_tail:A) */
> +
> + next_lpos = atomic_long_read(&data_ring->tail_lpos
> + ); /* LMM(data_push_tail:B) */
> + if (next_lpos == tail_lpos)
> + return false;
> +
> + /* Another task pushed the tail. Try again. */
> + tail_lpos = next_lpos;
> + continue;

The purpose of the continue is to try again _without_ attempting a
cmpxchg(). However, since the cmpxchg() is in the while condition, it
_is_ called. This is a major problem because if the cmpxchg() succeeds
(highly likely at this point) then the smp_mb() is not called and
success is returned even though the tail may not have been pushed far
enough.

The cmpxchg() needs to be moved out of the while condition so that a
continue can be used as intended. Patch below.

> + }
> +
> + /*
> + * Guarantee any descriptor states that have transitioned to
> + * reusable are stored before pushing the tail lpos. This
> + * allows readers to identify if data has expired while
> + * reading the descriptor. This pairs with desc_read:D.
> + */
> + smp_mb(); /* LMM(data_push_tail:C) */
> +
> + } while (!atomic_long_try_cmpxchg_relaxed(&data_ring->tail_lpos,
> + &tail_lpos, next_lpos)); /* LMM(data_push_tail:D) */
> +
> + return true;
> +}

John Ogness

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index e0a66468d4f3..ac542bdb8deb 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -546,11 +546,8 @@ static bool data_push_tail(struct printk_ringbuffer *rb,

tail_lpos = atomic_long_read(&data_ring->tail_lpos);

- do {
- /* Done, if the tail lpos is already at or beyond @lpos. */
- if ((lpos - tail_lpos) - 1 >= DATA_SIZE(data_ring))
- break;
-
+ /* Done, if the tail lpos is already at or beyond @lpos. */
+ while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
/*
* Make all descriptors reusable that are associated with
* data blocks before @lpos.
@@ -601,8 +598,12 @@ static bool data_push_tail(struct printk_ringbuffer *rb,
*/
smp_mb(); /* LMM(data_push_tail:C) */

- } while (!atomic_long_try_cmpxchg_relaxed(&data_ring->tail_lpos,
- &tail_lpos, next_lpos)); /* LMM(data_push_tail:D) */
+ if (atomic_long_try_cmpxchg_relaxed(&data_ring->tail_lpos,
+ &tail_lpos,
+ next_lpos)) { /* LMM(data_push_tail:D) */
+ break;
+ }
+ }

return true;
}