From: Waiman Long longman@redhat.com
commit b613c7f31476c44316bfac1af7cac714b7d6bef9 upstream.
A non-first waiter can potentially spin in the for loop of rwsem_down_write_slowpath() without sleeping but fail to acquire the lock even if the rwsem is free if the following sequence happens:
Non-first RT waiter First waiter Lock holder ------------------- ------------ ----------- Acquire wait_lock rwsem_try_write_lock(): Set handoff bit if RT or wait too long Set waiter->handoff_set Release wait_lock Acquire wait_lock Inherit waiter->handoff_set Release wait_lock Clear owner Release lock if (waiter.handoff_set) { rwsem_spin_on_owner((); if (OWNER_NULL) goto trylock_again; } trylock_again: Acquire wait_lock rwsem_try_write_lock(): if (first->handoff_set && (waiter != first)) return false; Release wait_lock
A non-first waiter cannot really acquire the rwsem even if it mistakenly believes that it can spin on OWNER_NULL value. If that waiter happens to be an RT task running on the same CPU as the first waiter, it can block the first waiter from acquiring the rwsem leading to live lock. Fix this problem by making sure that a non-first waiter cannot spin in the slowpath loop without sleeping.
Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent") Signed-off-by: Waiman Long longman@redhat.com Signed-off-by: Ingo Molnar mingo@kernel.org Tested-by: Mukesh Ojha quic_mojha@quicinc.com Reviewed-by: Mukesh Ojha quic_mojha@quicinc.com Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20230126003628.365092-2-longman@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/locking/rwsem.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-)
--- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -624,18 +624,16 @@ static inline bool rwsem_try_write_lock( */ if (first->handoff_set && (waiter != first)) return false; - - /* - * First waiter can inherit a previously set handoff - * bit and spin on rwsem if lock acquisition fails. - */ - if (waiter == first) - waiter->handoff_set = true; }
new = count;
if (count & RWSEM_LOCK_MASK) { + /* + * A waiter (first or not) can set the handoff bit + * if it is an RT task or wait in the wait queue + * for too long. + */ if (has_handoff || (!rt_task(waiter->task) && !time_after(jiffies, waiter->timeout))) return false; @@ -651,11 +649,12 @@ static inline bool rwsem_try_write_lock( } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
/* - * We have either acquired the lock with handoff bit cleared or - * set the handoff bit. + * We have either acquired the lock with handoff bit cleared or set + * the handoff bit. Only the first waiter can have its handoff_set + * set here to enable optimistic spinning in slowpath loop. */ if (new & RWSEM_FLAG_HANDOFF) { - waiter->handoff_set = true; + first->handoff_set = true; lockevent_inc(rwsem_wlock_handoff); return false; }