Messages in this thread | ![/](/images/icornerl.gif) | | From | John Ogness <> | Subject | Re: [PATCH printk v2 6/9] printk: Wait for all reserved records with pr_flush() | Date | Wed, 08 Nov 2023 12:14:54 +0106 |
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On 2023-11-06, John Ogness <john.ogness@linutronix.de> wrote: > diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c > index 7c121fd68330..dc83569d3a3a 100644 > --- a/kernel/printk/printk_ringbuffer.c > +++ b/kernel/printk/printk_ringbuffer.c > @@ -2012,6 +2012,102 @@ static u64 prb_first_seq(struct printk_ringbuffer *rb) > return seq; > } > > +/** > + * prb_next_reserve_seq() - Get the sequence number after the most recently > + * reserved record. > + * > + * @rb: The ringbuffer to get the sequence number from. > + * > + * This is the public function available to readers to see what sequence > + * number will be assigned to the next reserved record. > + * > + * Note that depending on the situation, this value can be equal to or > + * higher than the sequence number returned by prb_next_seq(). > + * > + * Context: Any context. > + * Return: The sequence number that will be assigned to the next record > + * reserved. > + */ > +u64 prb_next_reserve_seq(struct printk_ringbuffer *rb) > +{ > + struct prb_desc_ring *desc_ring = &rb->desc_ring; > + unsigned long last_finalized_id; > + atomic_long_t *state_var; > + u64 last_finalized_seq; > + unsigned long head_id; > + struct prb_desc desc; > + unsigned long diff; > + struct prb_desc *d; > + int err; > + > + /* > + * It may not be possible to read a sequence number for @head_id. > + * So the ID of @last_finailzed_seq is used to calculate what the > + * sequence number of @head_id will be. > + */ > + > +try_again: > + last_finalized_seq = desc_last_finalized_seq(desc_ring); > + > + /* > + * @head_id is loaded after @last_finalized_seq to ensure that it is > + * at or beyond @last_finalized_seq. > + * > + * Memory barrier involvement: > + * > + * If desc_last_finalized_seq:A reads from > + * desc_update_last_finalized:A, then > + * prb_next_reserve_seq:A reads from desc_reserve:D. > + * > + * Relies on: > + * > + * RELEASE from desc_reserve:D to desc_update_last_finalized:A > + * matching > + * ACQUIRE from desc_last_finalized_seq:A to prb_next_reserve_seq:A > + * > + * Note: desc_reserve:D and desc_update_last_finalized:A can be > + * different CPUs. However, the desc_update_last_finalized:A CPU > + * (which performs the release) must have previously seen > + * desc_read:C, which implies desc_reserve:D can be seen. > + */ > + head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_next_reserve_seq:A) */ > + > + d = to_desc(desc_ring, last_finalized_seq); > + state_var = &d->state_var; > + > + /* Extract the ID, used to specify the descriptor to read. */ > + last_finalized_id = DESC_ID(atomic_long_read(state_var)); > + > + /* Ensure @last_finalized_id is correct. */ > + err = desc_read_finalized_seq(desc_ring, last_finalized_id, last_finalized_seq, &desc); > + > + if (err == -EINVAL) { > + if (last_finalized_seq == 0) { > + /* > + * @last_finalized_seq still contains its initial > + * value. Probably no record has been finalized yet. > + * This means the ringbuffer is not yet full and the > + * @head_id value can be used directly (subtracting > + * off its initial value). > + * > + * Because of hack#2 of the bootstrapping phase, the > + * @head_id initial value must be handled separately. > + */ > + if (head_id == DESC0_ID(desc_ring->count_bits)) > + return 0; > + > + last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1; > + } else { > + /* Record must have been overwritten. Try again. */ > + goto try_again; > + } > + } > + > + diff = head_id - last_finalized_id; > + > + return (last_finalized_seq + diff);
This needs to be:
return (last_finalized_seq + diff + 1);
because @last_finalized_seq and @head_id refer to existing records, but this function should return the following (not yet existing) record.
John Ogness
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