Subject: Re: [RFC] arch: Introduce new TSO memory barrier smp_tmb()

From: Mathieu Desnoyers <[email-masked]>
Date: Thu, 7 Nov 2013 18:50:01 -0500

* Peter Zijlstra ([email-masked]) wrote:

[...]

Hi Peter,

Looking at this simplified version of perf's ring buffer
synchronization, I get concerned about the following issue:

> /*
>  * One important detail is that the kbuf part and the kbuf_writer() are
>  * strictly per cpu and we can thus rely on program order for those.
>  *
>  * Only the userspace consumer can possibly run on another cpu, and thus we
>  * need to ensure data consistency for those.
>  */
> 
> struct buffer {
>         u64 size;
>         u64 tail;
>         u64 head;
>         void *data;
> };
> 
> struct buffer *kbuf, *ubuf;
> 
> /*
>  * If there's space in the buffer; store the data @buf; otherwise
>  * discard it.
>  */
> void kbuf_write(int sz, void *buf)
> {
> 	u64 tail, head, offset;
> 
> 	do {
> 		tail = ACCESS_ONCE(ubuf->tail);
> 		offset = head = kbuf->head;
> 		if (CIRC_SPACE(head, tail, kbuf->size) < sz) {
> 			/* discard @buf */
> 			return;
> 		}
> 		head += sz;
> 	} while (local_cmpxchg(&kbuf->head, offset, head) != offset)
> 

Let's suppose we have a thread executing kbuf_write(), interrupted by an
IRQ or NMI right after a successful local_cmpxchg() (space reservation
in the buffer). If the nested execution context also calls kbuf_write(),
it will therefore update ubuf->head (below) with the second reserved
space, and only after that will it return to the original thread context
and continue executing kbuf_write(), thus overwriting ubuf->head with
the prior-to-last reserved offset.

All this probably works OK most of the times, when we have an event flow
guaranteeing that a following event will fix things up, but there
appears to be a risk of losing events near the end of the trace when
those are in nested execution contexts.

Thoughts ?

Thanks,

Mathieu

>         /*
>          * Ensure that if we see the userspace tail (ubuf->tail) such
>          * that there is space to write @buf without overwriting data
>          * userspace hasn't seen yet, we won't in fact store data before
>          * that read completes.
>          */
> 
>         smp_mb(); /* A, matches with D */
> 
>         memcpy(kbuf->data + offset, buf, sz);
> 
>         /*
>          * Ensure that we write all the @buf data before we update the
>          * userspace visible ubuf->head pointer.
>          */
>         smp_wmb(); /* B, matches with C */
> 
>         ubuf->head = kbuf->head;
> }
> 
> /*
>  * Consume the buffer data and update the tail pointer to indicate to
>  * kernel space there's 'free' space.
>  */
> void ubuf_read(void)
> {
>         u64 head, tail;
> 
>         tail = ACCESS_ONCE(ubuf->tail);
>         head = ACCESS_ONCE(ubuf->head);
> 
>         /*
>          * Ensure we read the buffer boundaries before the actual buffer
>          * data...
>          */
>         smp_rmb(); /* C, matches with B */
> 
>         while (tail != head) {
>                 obj = ubuf->data + tail;
>                 /* process obj */
>                 tail += obj->size;
>                 tail %= ubuf->size;
>         }
> 
>         /*
>          * Ensure all data reads are complete before we issue the
>          * ubuf->tail update; once that update hits, kbuf_write() can
>          * observe and overwrite data.
>          */
>         smp_mb(); /* D, matches with A */
> 
>         ubuf->tail = tail;
> }

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com