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

From: Eric Dumazet
Date: Thu Mar 08 2007 - 18:43:41 EST


Michael K. Edwards a Ãcrit :
from sys_write():

file = fget_light(fd, &fput_needed);
if (file) {
loff_t pos = file_pos_read(file);
ret = vfs_write(file, buf, count, &pos);
file_pos_write(file, pos);
fput_light(file, fput_needed);
}

Surely that's racy when two threads write to the same fd and the first
vfs_write call blocks or is preempted. Or does fget_light take some
per-file lock that I'm not seeing?

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.

Only O_APPEND case is specially handled (and NFS might fail to handle this case correctly)

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