Re: [PATCH 10/12] NFS: Do not serialise O_DIRECT reads and writes

[Trond Myklebust <trondmy@xxxxxxxxxxxxxxx>]

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.

â 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).
â The writes need protection against O_DIRECT, they donât need serialisation with other reads, but they do need to be serialised against other buffered writes.

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