Let $1\leq p,q,r\leq \infty$ such that $\frac{1}{r}=\frac{1}{p}+\frac{1}{q}$. Let $S_p$ denote the Schatten space. For any $x\in S_p$ and any $y\in S_q$ we have $$ ||xy||_{S_r} \leq ||x||_{S_p}||y||_{S_q} $$ (noncommutative Hölder's inequality).

Does it exists necessary and sufficient conditions on $x,y$ in order to have an equality in this inequality?

More generally, I ask the same question replacing $S_p$ by the noncommutative $L_p$-space $L_p(M)$ associated with a semifinite von Neumann algebra $M$ equipped with a normal semifinite faithful trace $\tau$.