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SubjectRe: [RFC PATCH v5 2/3] printk-rb: new printk ringbuffer implementation (reader)
On (19/12/09 17:43), Sergey Senozhatsky wrote:
> > +static int desc_read_committed(struct prb_desc_ring *desc_ring, u32 id,
> > + u64 seq, struct prb_desc *desc)
> > +{
> > + enum desc_state d_state;
> > +
> > + d_state = desc_read(desc_ring, id, desc);
> > + if (desc->info.seq != seq)
> > + return -EINVAL;
> > + else if (d_state == desc_reusable)
> > + return -ENOENT;
> > + else if (d_state != desc_committed)
> > + return -EINVAL;
> > +
> > + return 0;
> > +}
> > +
> > +/*
> > + * Copy the ringbuffer data from the record with @seq to the provided
> > + * @r buffer. On success, 0 is returned.
> > + *
> > + * See desc_read_committed() for error return values.
> > + */
> > +static int prb_read(struct printk_ringbuffer *rb, u64 seq,
> > + struct printk_record *r)
> > +{
> > + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> > + struct prb_desc *rdesc = to_desc(desc_ring, seq);
> > + atomic_t *state_var = &rdesc->state_var;
> > + struct prb_desc desc;
> > + int err;
> > + u32 id;
> > +
> > + /* Get a reliable local copy of the descriptor and check validity. */
> > + id = DESC_ID(atomic_read(state_var));
> > + err = desc_read_committed(desc_ring, id, seq, &desc);
> > + if (err)
> > + return err;
> > +
> > + /* If requested, copy meta data. */
> > + if (r->info)
> > + memcpy(r->info, &desc.info, sizeof(*(r->info)));
>
> I wonder if those WARN_ON-s will trigger false positive sometimes.
>
> A theoretical case.
>
> What if reader gets preempted/interrupted in the middle of
> desc_read_committed()->desc_read()->memcpy(). The context which interrupts
> the reader recycles the descriptor and pushes new data. Suppose that
> reader was interrupted right after it copied ->info.seq and ->info.text_len.
> So the first desc_read_committed() will pass - we have matching ->seq
> and committed state. copy_data(), however, most likely, will generate
> WARNs. The final desc_read_committed() will notice that local copy
> of desc was in non-consistent state and everything is fine, but we have
> WARNs in the log buffer now.

Hmm. No, that won't happen. We should get desc_miss first, and then -EINVAL.

-ss

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