On Thu, Apr 11, 2019 at 5:38 AM Kirill Smelkov <kirr@xxxxxxxxxx> wrote:
>
> However file->f_pos writing is still there and it will bug under race
> detector, e.g. under KTSAN because read and write can be running
> simultaneously. Maybe it is not only race bug, but also a bit of
> slowdown as read and write code paths write to the same memory thus
> needing inter-cpu synchronization if read and write handlers are on
> different cpus. However for this I'm not sure.
I doubt it's noticeable, but it would probably be good to simply not
do the write for streams.
That *could* be done somewhat similarly, with just changing th address
to be updated, or course.
In fact, we *used* to (long ago) pass in the address of "file->f_pos"
itself to the low-level read/write routines. We then changed it to do
that indirection through a local copy of pos (and
file_pos_read/file_pos_write) because we didn't do the proper locking,
so different read/write versions could mess with each other (and with
lseek).
But one of the things that commit 9c225f2655e36 ("vfs: atomic f_pos
accesses as per POSIX") did was to add the proper locking at least for
the cases that we care about deeply, so we *could* say that we have
three cases:
- FMODE_ATOMIC_POS: properly locked,
- FMODE_STREAM: no pos at all
- otherwise a "mostly don't care - don't mix!"
and so we could go back to not copying the pos at all, and instead do
something like
loff_t *ppos = f.file->f_mode & FMODE_STREAM ? NULL : &file->f_pos;
ret = vfs_write(f.file, buf, count, ppos);
and perhaps have a long-term plan to try to get rid of the "don't mix"
case entirely (ie "if you use f_pos, then we'll do the proper
locking")
(The above is obviously surrounded by the fdget_pos()/fdput_pos() that
implements the locking decision).
Linus
