Queued ticket lock description in English and some RISC-V assembly for a possible implementation

ticket-lock-riscv.s

#pragma once

#  Ticket Lock
# 
#  Ticket locks are fair spinlocks that order acccess on a first-in,
#  first-out basis. The lock is composed of a head counter and a tail
#  counter. The head counter indicates the ticket number of the current
#  lock owner. The tail counter indicates the last issued ticket number.
#  To acquire the lock, the acquiring thread atomically increments the
#  tail counter to assign itself a ticket number. It then waits until
#  its assigned ticket number is seen in the head counter. If the lock
#  is not contended, then head will equal the ticket number just assigned.
#  To release the lock, the lock owner atomically increments head counter
#  thereby allowing access to the thread holding the next ticket number.
# 
#  Initializtion - the lock is initialized unlocked, so that the
#  next tail increment returns a ticket that will acquires the lock.
# 
#    tail   | head
#    -------|-------
#    0x0000 | 0x0001
# 
#  Lock - the tail is incremented, and the thread waits unti the tail
#  is equal to the head before returning control, which is the typical
#  case if the lock is uncontended.
# 
#    tail   | head
#    -------|-------
#    0x0001 | 0x0001
# 
#  Unlock - the head is incremented, so that it will now equal the value
#  of the ticket of the next waiter, if any.
# 
#    tail   | head
#    -------|-------
#    0x0001 | 0x0002
# 
#  Waiters - the number of waiters can be calculated by subtracting
#  head (current ticket) from the tail plus one (next ticket)
# 
#    num_waiters = tail + 1 - head
# 
 
spin_lock:
    lui     a2,0x10
    amoadd.w a5,a5,(a0)
    srliw   a4,a5,0x10
2:  slliw   a5,a5,0x10
    srliw   a5,a5,0x10
    bne     a5,a4,3f
    ret
3:  lw      a5,0(a0)
    fence   r,rw
    j       2b

spin_trylock:
    lui     a5,0x10
    lr.w.aq a4,(a0)
    addw    a5,a5,a4
    slliw   a3,a5,0x10
    srliw   a3,a3,0x10
    srliw   a4,a5,0x10
    beq     a3,a4,2f
1:  li      a0,0
    ret
2:  sc.w.rl a4,a5,(a0)
    bnez    a4,1b
    fence   r,rw
    li      a0,1
    ret

spin_unlock:
    fence   rw,w
1:  lr.w.aq a4,(a0)
    srliw   a5,a4,0x10
    addiw   a4,a4,1
    slliw   a4,a4,0x10
    srliw   a4,a4,0x10
    slliw   a5,a5,0x10
    or      a5,a5,a4
    sc.w.rl a4,a5,(a0)
    bnez    a5,1b
    ret