Regarding mmap synchronization.

Dave Hylands dhylands at
Sun Sep 18 15:56:45 EDT 2011


On Sun, Sep 18, 2011 at 12:12 PM, mindentropy <mindentropy at> wrote:
> On Monday 19 Sep 2011 12:19:29 AM Dave Hylands wrote:
>> The way I normally deal with this is to use 2 indicies, a get index
>> and a put index. One of the indicies if only ever written by kernel
>> space, and the other is only ever written by user space.
> That is the setup I have now.
>> You make the circular buffer be a power of 2 in size, and you
>> determine the number of items in the queue by subtracting the get
>> index from the put index.
> I am worried about the subtraction. Is it safe to subtract when the put index
> is in the process of incrementing by the kernel?
> My queue size is always a power of two and avoid the modulus operation by '&'
> with (2^n)-1.

The worst case that happens is that you miss the increment by the
other side. This means that you either miss a new item added onto the
queue (if you're reading) or you miss new space added to the queue (if
you're writing). Either case is non-fatal. You don't corrupt anything.
You'll pick up the new item/space the next time you check.

>> If the items in the circular buffer are in cached memory, then I
>> normally try to make each item be an exact multiple of the cache line
>> size. I find using uncached memory is generally better for this type
>> of thing (the accesses are slower, but may be faster after accounting
>> for the cache management).
> I am having a chunked buffer. So the queue items are memory chunks ranging from
> PAGE_SIZE*n to PAGE_SIZE*1. So I have a index to the chunk, index to item
> inside the chunk for read and write. For the user its invisible and sees it as
> one big chunk of memory with a read and a write index.

Seems reasonable. The big thingis to ensure that when you add an item
to the queue, that the item is written fully before incrementing the
index. If both the memory for the items and the memory for the index
is uncached (and declared volatile), then this should be fine.

Dave Hylands
Shuswap, BC, Canada

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