Michael K. Edwards a écrit :
On 3/8/07, Eric Dumazet <[EMAIL PROTECTED]> 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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