Date: Thu, 07 May 2015 08:20:58 -0600 From: Ian Lepore <ian@freebsd.org> To: rhenjau <rhenjau@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Memory barriers about buf_ring(9) Message-ID: <1431008458.6170.165.camel@freebsd.org> In-Reply-To: <CA%2BO7MXzRuy33tWQRWAnh3Z6ZsdLOsVvm0-1p8%2BjrfFOannjZbQ@mail.gmail.com> References: <CA%2BO7MXzRuy33tWQRWAnh3Z6ZsdLOsVvm0-1p8%2BjrfFOannjZbQ@mail.gmail.com>
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On Thu, 2015-05-07 at 18:10 +0800, rhenjau wrote:
> Hi, hackers
> I'm reading buf_ring(9) and have a question about the following
> function. Is a memory barrier needed before br_cons_tail to prevent the
> loading of br_ring[cons_head] load is reordered after br_cons_tail is set?
> Producers may overwrite the area.
>
> /*
> * single-consumer dequeue
> * use where dequeue is protected by a lock
> * e.g. a network driver's tx queue lock
> */
> static __inline void *
> buf_ring_dequeue_sc(struct buf_ring *br)
> {
> uint32_t cons_head, cons_next;
> #ifdef PREFETCH_DEFINED
> uint32_t cons_next_next;
> #endif
> uint32_t prod_tail;
> void *buf;
>
> cons_head = br->br_cons_head;
> prod_tail = br->br_prod_tail;
>
> cons_next = (cons_head + 1) & br->br_cons_mask;
> #ifdef PREFETCH_DEFINED
> cons_next_next = (cons_head + 2) & br->br_cons_mask;
> #endif
>
> if (cons_head == prod_tail)
> return (NULL);
>
> #ifdef PREFETCH_DEFINED
> if (cons_next != prod_tail) {
> prefetch(br->br_ring[cons_next]);
> if (cons_next_next != prod_tail)
> prefetch(br->br_ring[cons_next_next]);
> }
> #endif
> br->br_cons_head = cons_next;
> buf = br->br_ring[cons_head];
>
> #ifdef DEBUG_BUFRING
> br->br_ring[cons_head] = NULL;
> if (!mtx_owned(br->br_lock))
> panic("lock not held on single consumer dequeue");
> if (br->br_cons_tail != cons_head)
> panic("inconsistent list cons_tail=%d cons_head=%d",
> br->br_cons_tail, cons_head);
> #endif
> ----------------------------------------------------- need memory
> barrier?
> br->br_cons_tail = cons_next;
> return (buf);
> }
>
There was some discussion and changes relating to barriers in buf_ring,
but it looks like the changes never got committed (I don't know why).
https://reviews.freebsd.org/D1945
-- Ian
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