netdev - Re: Question on ptr

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Date:   Fri, 1 Feb 2019 16:12:46 +0800
From:   fei phung <feiphung27@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     feiphung@...mail.com, netdev@...r.kernel.org
Subject: Re: Question on ptr_ring linux header

> I am not sure what does assignment of pointers mean in this context.
> ptr_ring is designed for a single producer and a single consumer.  For
> why it works see explanation about data dependencies in
> Documentation/memory-barriers.txt.  You will have to be more specific
> about the data race that you see if you expect more specific answers.

Hi,

ptr_ring_produce_any(sc->recv[chnl]->msgs,
&item_recv_push[item_recv_push_index])  needs to have
non-NULL pointer assigned for item_recv_push[item_recv_push_index] , right ?

Note:
ptr_ring_produce_any() occurs in interrupt handler, while
ptr_ring_consume_any() occurs in thread.

https://gist.github.com/promach/7716ee8addcaa33fda140d74d1ad94d6/cdc6599b8313e265bdfb073a65a124e1ba3303a2#file-riffa_driver_ptr_ring-c-L306-L320

        // TX (PC receive) scatter gather buffer is read.
        if (vect & (1<<((5*i)+1))) {
            recv = 1;

            item_recv_push[item_recv_push_index].val1 = EVENT_SG_BUF_READ;
            item_recv_push[item_recv_push_index].val2 = 0;

            // Keep track so the thread can handle this.
            if (ptr_ring_produce_any(sc->recv[chnl]->msgs,
&item_recv_push[item_recv_push_index])) {
                printk(KERN_ERR "riffa: fpga:%d chnl:%d, recv sg buf
read msg queue full\n", sc->id, chnl);
            }
            DEBUG_MSG(KERN_INFO "riffa: fpga:%d chnl:%d, recv sg buf
read\n", sc->id, chnl);

            item_recv_push_index++;
        }

The kernel log points me to ptr_ring_consume_any(). So, this is
definitely data race issue
with my own ptr_ring interfacing code.

besides, I am also getting a reference to zero-length ring for the
kernel dmesg log.
I am not sure how this is related to the data race though.

The kernel log points to
https://github.com/torvalds/linux/blame/master/include/linux/ptr_ring.h#L175
(click open git blame on this line)

Regards,
Phung