Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

[Tom Zanussi <tzanussi@gmail.com>]

Hi,

On Fri, 2008-06-13 at 04:09 +0300, Eduard - Gabriel Munteanu wrote:
> Suppose CPU0, as instructed by userspace, reads CPU1's data. If the
> latter is logging data, it's not enough to disable IRQs or preemption
> to protect the data. Added a per-buffer (thus per-CPU) spinlock to
> prevent concurrent access. The choice of using a spinlock is motivated
> by the need to log data (and thus lock the buffer) from interrupt
> context. The problem was revealed when working on kmemtrace, where some
> events were seemingly out-of-order or just all-zeros, even though the
> necessary precautions had already been taken.
> 

Alternatively, you could get rid of the problem by making sure CPU0
never reads CPU1's data, by having the userspace reader use per-cpu
threads and using sched_setaffinity() to pin each thread to a given cpu.
See for example, the blktrace code, which does this.

Actually, in a few days or so I'm planning on releasing the first cut of
a library that makes this and all the rest of the nice blktrace
userspace code available to other tracing applications, not just
blktrace.  Hopefully it would be something that you'd be able to use for
kmemtrace as well; in that case, you'd just use the library and not have
to worry about these details.

Tom


> Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
> ---
>  include/linux/relay.h |   10 +++++++---
>  kernel/relay.c        |   11 ++++++++++-
>  2 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/relay.h b/include/linux/relay.h
> index 8593ca1..a3a03e7 100644
> --- a/include/linux/relay.h
> +++ b/include/linux/relay.h
> @@ -38,6 +38,7 @@ struct rchan_buf
>  	size_t subbufs_produced;	/* count of sub-buffers produced */
>  	size_t subbufs_consumed;	/* count of sub-buffers consumed */
>  	struct rchan *chan;		/* associated channel */
> +	spinlock_t rw_lock;		/* protects buffer during R/W */
>  	wait_queue_head_t read_wait;	/* reader wait queue */
>  	struct timer_list timer; 	/* reader wake-up timer */
>  	struct dentry *dentry;		/* channel file dentry */
> @@ -200,13 +201,14 @@ static inline void relay_write(struct rchan *chan,
>  	unsigned long flags;
>  	struct rchan_buf *buf;
>  
> -	local_irq_save(flags);
> -	buf = chan->buf[smp_processor_id()];
> +	buf = chan->buf[get_cpu()];
> +	spin_lock_irqsave(&buf->rw_lock, flags);
>  	if (unlikely(buf->offset + length >= chan->subbuf_size))
>  		length = relay_switch_subbuf(buf, length);
>  	memcpy(buf->data + buf->offset, data, length);
>  	buf->offset += length;
> -	local_irq_restore(flags);
> +	spin_unlock_irqrestore(&buf->rw_lock, flags);
> +	put_cpu();
>  }
>  
>  /**
> @@ -228,10 +230,12 @@ static inline void __relay_write(struct rchan *chan,
>  	struct rchan_buf *buf;
>  
>  	buf = chan->buf[get_cpu()];
> +	spin_lock(&buf->rw_lock);
>  	if (unlikely(buf->offset + length >= buf->chan->subbuf_size))
>  		length = relay_switch_subbuf(buf, length);
>  	memcpy(buf->data + buf->offset, data, length);
>  	buf->offset += length;
> +	spin_unlock(&buf->rw_lock);
>  	put_cpu();
>  }
>  
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 07f25e7..250a27a 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -430,6 +430,8 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
>  	if (!buf)
>   		goto free_name;
>  
> +	spin_lock_init(&buf->rw_lock);
> +
>   	buf->cpu = cpu;
>   	__relay_reset(buf, 1);
>  
> @@ -1013,11 +1015,13 @@ static ssize_t relay_file_read_subbufs(struct file *filp, loff_t *ppos,
>  	struct rchan_buf *buf = filp->private_data;
>  	size_t read_start, avail;
>  	int ret;
> +	unsigned long flags;
>  
>  	if (!desc->count)
>  		return 0;
>  
>  	mutex_lock(&filp->f_path.dentry->d_inode->i_mutex);
> +	spin_lock_irqsave(&buf->rw_lock, flags);
>  	do {
>  		if (!relay_file_read_avail(buf, *ppos))
>  			break;
> @@ -1028,15 +1032,20 @@ static ssize_t relay_file_read_subbufs(struct file *filp, loff_t *ppos,
>  			break;
>  
>  		avail = min(desc->count, avail);
> +		/* subbuf_actor may sleep, so release the spinlock for now */
> +		spin_unlock_irqrestore(&buf->rw_lock, flags);
>  		ret = subbuf_actor(read_start, buf, avail, desc, actor);
>  		if (desc->error < 0)
> -			break;
> +			goto out;
> +		spin_lock_irqsave(&buf->rw_lock, flags);
>  
>  		if (ret) {
>  			relay_file_read_consume(buf, read_start, ret);
>  			*ppos = relay_file_read_end_pos(buf, read_start, ret);
>  		}
>  	} while (desc->count && ret);
> +	spin_unlock_irqrestore(&buf->rw_lock, flags);
> +out:
>  	mutex_unlock(&filp->f_path.dentry->d_inode->i_mutex);
>  
>  	return desc->written;