Re: do_loop_readv_writev() not as described for drivers implementing only write()?

From: Daniel Baluta
Date: Wed Mar 03 2010 - 07:11:35 EST

On Tue, Mar 2, 2010 at 2:55 PM, Mike McTernan <mmcternan@xxxxxxxxxxx> wrote:
> Hi,
> I'm using writev() with an old FPGA driver which only implements
> write(), not aio_write().  I'm expecting the behaviour described in the
> man page for writev():
> "The  data  transfers  performed by readv() and writev() are atomic: the
> data written by writev() is written as a single block that is not
> intermingled with output from writes in other processes (but see pipe(7)
> for an exception); analogously, readv() is guaranteed to read a
> contiguous block of data from the file, regardless of read operations
> performed in other threads or processes that have file descriptors
> referring to the same open file description (see open(2))."
> I appear to be observing intermingling of individual iovec entries that
> are being written to the same fd from different threads i.e. each call
> to writev() isn't producing a contiguous block to be output.  This is at
> odds with the man page description.
> Looking into the kernel sources (from around 2.6.28 to 2.6.33), the
> driver doesn't implement aio_write(), so vfs_writev() gets handled by
> do_loop_readv_writev() as a series of discrete calls to the driver's
> write().
> I can't see where any locking is applied to ensure each iovec is handled
> serially without 'internmingling', which would awkwardly have to be
> outside the driver in this case.

I'm also interested about this topic. Can anyone help?
I would say that the libc enforces atomicity, but I have to dig deeper for
a real answer :).

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