How to implement a driver's read and write operations with synchronization properly

[Jonathan Neuschäfer]

On Tue, Jul 29, 2014 at 03:03:55PM +0700, Anh Le wrote:
> Hi everyone,
> I'm trying to write a misc char driver with read and write operations,
> and I choose reader writer lock for synchronization between them.
> Suppose that when writing 2000 bytes, the write operation is called
> twice, each time writing only 1000 bytes. There are 2 ways to
> implement synchronization as below:

Which "write operation" is called twice in your scenario?

> 1. Acquire and release lock everytime the write operation is called.
> Same with read operation.
> 2. Acquire the lock when the write operation is called for the first
> time, and release when the whole write procedure is completed. In this
> case, the lock is acquired before transfering the 1st byte, and
> released after transfering the 2000th byte. Same with read operation.

If a userspace program writes 1000 bytes at first, how can you know that
it wants to perform another write later on?

> As how I see it, both of these approaches have problems. The first one
> release the lock in between the write operations, so a reader can get
> in while the writer has only finished a portion of its work.

If a userspace program wants to write a chunk of data atomically, it
should use just one call to write(2). (On Linux, one can save some
copying by using writev(2), which writes data from multiple buffers in
one atomic step.)

> With the
> second approach, I have no way to tell that the write procedure ends
> with the 2000th byte. What if there are more?

Again, where does this write procedure run?


Greetings,
Jonathan Neuschäfer

> Could anyone tell me the proper way to deal with this situation?
> Thanks in advance.
> 
> -- 
> Le Quoc Anh