Re: VFS locking: f_pos thread-safe ?

[Matthias Urlichs <smurf@smurf.noris.de>]

Hi, Joel Becker wrote:

> This reads:  "all of the specified effects of the other call,
> or none of them."  If I read that correctly, if f_pos is at N, and
> threads A and B concurrently read M bytes, then each thread's read()
> must either start at f_pos = N or f_pos = N+M, but never at N < f_pos <
> N+M.  So as long as our code doesn't partially update f_pos, it is
> valid.

Umm, strictly speaking there are three possible valid cases:

thread A reads M @N   thread B reads M @N     file pointer ends up as N+M
thread A reads M @N   thread B reads M @M+N   file pointer ends up as N+2M
thread A reads M @M+N thread B reads M @N     file pointer ends up as N+2M

With your description,
thread A reads M @M+N thread B reads M @M+N   file pointer ends up as N+2M

would be equally valid, which I'd declare buggy ^W non-conforming.

-- 
Matthias Urlichs