On Sat, 30 Nov 2024 at 04:30, Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> What does the smp_load_acquire() pair with?
I'm not sure we have them everywhere, but at least this one at dentry
creation time.
__d_alloc():
/* Make sure we always see the terminating NUL character */
smp_store_release(&dentry->d_name.name, dname); /* ^^^ */
so even at rename time, when we swap the d_name.name pointers
(*without* using a store-release at that time), both of the dentry
names had memory orderings before.
That said, looking at swap_name() at the non-"swap just the pointers"
case, there we do just "memcpy()" the name, and it would probably be
good to update the target d_name.name with a smp_store_release.
In practice, none of this ever matters. Anybody who uses the dentry
name without locking either doesn't care enough (like comm[]) or will
use the sequence number thing to serialize at a much higher level. So
the smp_load_acquire() could probably be a READ_ONCE(), and nobody
would ever see the difference.
Linus
