Eric Dumazet a écrit :

Al Viro a écrit :

quoted

On Thu, Nov 27, 2008 at 12:32:59AM +0100, Eric Dumazet wrote:

quoted

This function arms a flag (MNT_SPECIAL) on the vfs, to avoid
refcounting on permanent system vfs.
Use this function for sockets, pipes, anonymous fds.

IMO that's pushing it past the point of usefulness; unless you can show
that this really gives considerable win on pipes et.al. *AND* that it
doesn't hurt other loads...

Well, if this is the last cache line that might be shared, then yes, 
numbers can talk.
But coming from 10 to 1 instead of 0 is OK I guess

quoted

dput() part: again, I want to see what happens on other loads; it's 
probably
fine (and win is certainly more than from mntput() change), but...  The
thing is, atomic_dec_and_lock() in there is often done on dentries with
d_count > 1 and that's fairly cheap (and doesn't involve contention on
dcache_lock on sane targets).

FWIW, unless there's a really good reason to do alpha 
atomic_dec_and_lock()
in a special way, I'd try to compare with

quoted

        if (atomic_add_unless(&dentry->d_count, -1, 1))
                return;

I dont know, but *reading* d_count before trying to write it is expensive
on modern cpus. Oprofile clearly show that on Intel Core2.

Then, *testing* the flag before doing the atomic_something() has the same
problem. Or we should put flag in a different cache line.

I am lazy (time for a sleep here), maybe we are smart here and use a 
trick like that already ?

atomic_t atomic_read_with_write_intent(atomic_t *v)
{
       int val = 0;
    /*
     * No LOCK prefix here, we only give a write intent hint to cpu
     */
       asm volatile("xaddl %0, %1"
                    : "+r" (val), "+m" (v->counter)
                    : : "memory");
       return val;
}

Forget it, its wrong... I really need to sleep :)