On 6/15/16, 10:56, "Christoph Hellwig" <hch@xxxxxxxxxxxxx> wrote:
>On Wed, Jun 15, 2016 at 02:52:24PM +0000, Trond Myklebust wrote:
>> >But XFS allows full parallelism for direct reads and writes as long
>> >as there is no more pagecache to flush. But if you have pages in
>> >the pagecache you need the exclusive lock to prevent against new
>> >pagecache pages being added.
>>
>> Exactly. So does this.
>
>So let's avoid bloating the inode with another rw_semaphore, and make
>everyones life easier by using the existing lock.
As I said earlier, the problem with that is you end up artificially serialising buffered reads and buffered writes.
• The reads only need a shared lock in order to protect the I/O mode from flipping to O_DIRECT (and relying on page locks to protect against buffered writes).
• The writes need protection against O_DIRECT, they don’t need serialisation with other reads, but they do need to be serialised against other buffered writes.
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