On Thu, Sep 16, 2010 at 03:30:56PM +0100, David Howells wrote: > Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > > Is the rmb() really needed? > > > > Take this code from fs/namei.c for example: > > > > inode = next.dentry->d_inode; > > if (!inode) > > goto out_dput; > > > > if (inode->i_op->follow_link) { > > > > It happily dereferences dentry->d_inode without a barrier after > > checking it for non-null, while that d_inode might have just been > > initialized on another CPU with a freshly created inode. There's > > absolutely no synchornization with that on this side. > > Perhaps it's not necessary; once set, how likely is i_op to be changed once > I_NEW is cleared? Are the path_get()s protecting this? If there is no protection, then something like rcu_dereference() is needed for the assignment from next.dentry->d_inode. > > Isn't the fact that we check the pointer for being non-null (together > > with locking/barrier on the other side) enough to ensure that it's > > safe to dereference it? > > It's possible that since there's a dependency between the variables on the > reading CPU that the barrier is not required. > > I think I have to refer that question to Paul. We would need either one of the rcu_dereference() or smp_read_barrier_depends() APIs to enforce the dependency, for example, against the compiler. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html