On Wed, May 2, 2018 at 10:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: >> On Wed, May 02, 2018 at 03:53:47AM +0000, Daniel Colascione wrote: > > Suppose we make a userspace mutex implemented with a lock word having three > > bits: acquired, sleep_mode, and wait_pending, with the rest of the word not > > being relevant at the moment. > So ideally we'd kill FUTEX_WAIT/FUTEX_WAKE for mutexes entirely, and go > with FUTEX_LOCK/FUTEX_UNLOCK that have the same semantics as the > existing FUTEX_LOCK_PI/FUTEX_UNLOCK_PI, namely, the word contains the > owner TID. That doesn't work if you want to use the rest of the word for something else, like a recursion count. With FUTEX_WAIT and FUTEX_WAKE, you can make a lock with two bits. > As brought up in the last time we talked about spin loops, why do we > care if the spin loop is in userspace or not? Aside from the whole PTI > thing, the syscall cost was around 150 cycle or so, while a LOCK CMPXCHG > is around 20 cycles. So ~7 spins gets you the cost of entry. That's pre-KPTI, isn't it? -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
