locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath

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>

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index bd1d714a7ea08bc876fb5c60a61237e04d160b5b..de375feada514ced1d4e455b2e2dbeef2039c0e3 100644 (file)
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -586,18 +586,16 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
                         */
                        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;
@@ -613,11 +611,12 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
        } 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;
        }