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SubjectRe: [PATCH 1/3] relay - clean up subbuf switch
On Tue, 23 Sep 2008 00:27:02 -0500
Tom Zanussi <zanussi@comcast.net> wrote:

> Clean up relay_switch_subbuf() and make waking up consumers optional.
>
> Over time, relay_switch_subbuf() has accumulated some cruft - this
> patch cleans it up and at the same time makes available some of it
> available as common functions that any subbuf-switch implementor would
> need (this is partially in preparation for the next patch, which makes
> the subbuf-switch function completely replaceable). It also removes
> the hard-coded reader wakeup and moves it into a replaceable callback
> called notify_consumers(); this allows any given tracer to implement
> consumer notification as it sees fit.
>
> Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
>
> diff --git a/include/linux/relay.h b/include/linux/relay.h
> index 953fc05..17f0515 100644
> --- a/include/linux/relay.h
> +++ b/include/linux/relay.h
> @@ -159,6 +159,15 @@ struct rchan_callbacks
> * The callback should return 0 if successful, negative if not.
> */
> int (*remove_buf_file)(struct dentry *dentry);
> +
> + /*
> + * notify_consumers - new sub-buffer available, let consumers know
> + * @buf: the channel buffer
> + *
> + * Called during sub-buffer switch. Applications which don't
> + * want to notify anyone should implement an empty version.
> + */
> + void (*notify_consumers)(struct rchan_buf *buf);
> };

Does this comment format and placement get properly processed by the
kerneldoc tools?

> /*
> @@ -186,6 +195,48 @@ extern size_t relay_switch_subbuf(struct rchan_buf *buf,
> size_t length);
>
> /**
> + * relay_event_toobig - is event too big to fit in a sub-buffer?
> + * @buf: relay channel buffer
> + * @length: length of event
> + *
> + * Returns 1 if too big, 0 otherwise.
> + *
> + * switch_subbuf() helper function.
> + */
> +static inline int relay_event_toobig(struct rchan_buf *buf, size_t length)
> +{
> + return length > buf->chan->subbuf_size;
> +}
> +
> +/**
> + * relay_update_filesize - increase relay file i_size by length
> + * @buf: relay channel buffer
> + * @length: length to add
> + *
> + * switch_subbuf() helper function.
> + */
> +static inline void relay_update_filesize(struct rchan_buf *buf, size_t length)
> +{
> + if (buf->dentry)
> + buf->dentry->d_inode->i_size += length;
> + else
> + buf->early_bytes += length;
> +
> + smp_mb();
> +}

What locking protects the non-atomic modification of the 64-bit i_size
here?

> +/**
> + * relay_inc_produced - increase number of sub-buffers produced by 1
> + * @buf: relay channel buffer
> + *
> + * switch_subbuf() helper function.
> + */
> +static inline void relay_inc_produced(struct rchan_buf *buf)
> +{
> + buf->subbufs_produced++;
> +}

This also needs caller-provided locking. That's part of the function's
interface and should be documented,

> +/**
> * relay_write - write data into the channel
> * @chan: relay channel
> * @data: data to be written
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 8d13a78..53652f1 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -324,6 +324,21 @@ static int remove_buf_file_default_callback(struct dentry *dentry)
> return -EINVAL;
> }
>
> +/*
> + * notify_consumers() default callback.
> + */
> +static void notify_consumers_default_callback(struct rchan_buf *buf)
> +{
> + if (waitqueue_active(&buf->read_wait))
> + /*
> + * Calling wake_up_interruptible() from here
> + * will deadlock if we happen to be logging
> + * from the scheduler (trying to re-grab
> + * rq->lock), so defer it.
> + */
> + __mod_timer(&buf->timer, jiffies + 1);
> +}
> +
> /* relay channel default callbacks */
> static struct rchan_callbacks default_channel_callbacks = {
> .subbuf_start = subbuf_start_default_callback,
> @@ -331,6 +346,7 @@ static struct rchan_callbacks default_channel_callbacks = {
> .buf_unmapped = buf_unmapped_default_callback,
> .create_buf_file = create_buf_file_default_callback,
> .remove_buf_file = remove_buf_file_default_callback,
> + .notify_consumers = notify_consumers_default_callback,
> };
>
> /**
> @@ -508,6 +524,8 @@ static void setup_callbacks(struct rchan *chan,
> cb->create_buf_file = create_buf_file_default_callback;
> if (!cb->remove_buf_file)
> cb->remove_buf_file = remove_buf_file_default_callback;
> + if (!cb->notify_consumers)
> + cb->notify_consumers = notify_consumers_default_callback;
> chan->cb = cb;
> }
>
> @@ -726,30 +744,17 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
> void *old, *new;
> size_t old_subbuf, new_subbuf;
>
> - if (unlikely(length > buf->chan->subbuf_size))
> + if (unlikely(relay_event_toobig(buf, length)))
> goto toobig;
>
> if (buf->offset != buf->chan->subbuf_size + 1) {
> buf->prev_padding = buf->chan->subbuf_size - buf->offset;
> old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
> buf->padding[old_subbuf] = buf->prev_padding;
> - buf->subbufs_produced++;
> - if (buf->dentry)
> - buf->dentry->d_inode->i_size +=
> - buf->chan->subbuf_size -
> - buf->padding[old_subbuf];
> - else
> - buf->early_bytes += buf->chan->subbuf_size -
> - buf->padding[old_subbuf];
> - smp_mb();
> - if (waitqueue_active(&buf->read_wait))
> - /*
> - * Calling wake_up_interruptible() from here
> - * will deadlock if we happen to be logging
> - * from the scheduler (trying to re-grab
> - * rq->lock), so defer it.
> - */
> - __mod_timer(&buf->timer, jiffies + 1);
> + relay_inc_produced(buf);
> + relay_update_filesize(buf, buf->chan->subbuf_size -
> + buf->padding[old_subbuf]);
> + buf->chan->cb->notify_consumers(buf);
> }
>
> old = buf->data;
> @@ -763,7 +768,7 @@ size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
> buf->data = new;
> buf->padding[new_subbuf] = 0;
>
> - if (unlikely(length + buf->offset > buf->chan->subbuf_size))
> + if (unlikely(relay_event_toobig(buf, length + buf->offset)))
> goto toobig;

I think you can put the unlikely() into relay_event_toobig() and gcc
will dtrt. If that has any value.

> return length;
>


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