Re: sys_write() racy for multi-threaded append?

[Eric Dumazet]

Michael K. Edwards a Ãcrit :
> On 3/8/07, Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote:
> > Nothing in the manuals says that write() on same fd should be non racy 
> > : In
> > particular file pos might be undefined. There is a reason pwrite() 
> > exists.
> >
> > Kernel doesnt have to enforce thread safety as standard is quite clear.
>
> I know the standard _allows_ us to crash and burn (well, corrupt
> f_pos) when two threads race on an fd, but why would we want to?
> Wouldn't it be better to do something at least slightly sane, like add
> atomically to f_pos the expected number of number of bytes written,
> then do the write, then fix it up (again atomically) if vfs_write
> returns an unexpected pos?

Absolutely not. We dont want to slow down kernel 'just in case a fool might 
want to do crazy things'


> > Only O_APPEND case is specially handled (and NFS might fail to handle 
> > this
> > case correctly)
>
> Is it?  How?
mm/filemap.c

generic_write_checks()

if (file->f_flags & O_APPEND)
	*pos = i_size_read(inode);

done while inode is locked.

O_APPEND basically says : Just ignore fpos and always use the 'end of file'

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