[BUG] 2.2.1[78] : RTNETLINK lock not properly locking ?

[Willy Tarreau <willy.lkml@free.fr>]

Hello !

while I was searching how to implement an rtnl_lock() in the bonding code,
I discovered that the rtnl_shlock() function in 2.2.1[78] could misbehave if
CONFIG_RTNETLINK is not set :
   - it will nearly never allow concurrent accesses (seems to be what was
     intented when it was written)
   - it will not always prevent concurrent accesses, which is weird because
     rtnl_lock() only relies on rtnl_shlock() (and exlock, which is empty) to
     protect sensible areas

The first case is trivial : one at a time.
(code taken from include/linux/rtnetlink.h, line 639)

     while (atomic_read(&rtnl_rlockct))
            sleep_on(&rtnl_wait);
     atomic_inc(&rtnl_rlockct);

The second case isn't trivial, so I will quote some points in the code :

[rtnl_shlock]
(1) ---------
        while (atomic_read(&rtnl_rlockct))
(2) ---------
                sleep_on(&rtnl_wait);
(3) ---------
        atomic_inc(&rtnl_rlockct);
(4) ---------

[rtnl_shunlock]
(5) ---------
        if (atomic_dec_and_test(&rtnl_rlockct))
(6) ---------
                wake_up(&rtnl_wait);
(7) ---------

Consider 3 concurrent threads A, B and C.
- First, A needs the lock. Noone has it. It enters (1), then (3), sets the
  rtnl_rlockct to 1 and exits at (4).
- now B comes in (1). The lock is already set by A, so B goes to (2) and
  sleeps.
- A unlocks. It goes to (5), then (6)
- at this moment, C tries to lock in (1), an succeeds since A has just released
  the lock. So it gets the lock and goes to (3), then (4).
- A is at (6) and wakes B up and steps to (7) and exits.
- B is woken up and goes to (3) then (4).

=> B and C both have the lock.

Perhaps I have missed something, but I don't find what. If I'm right, then why
don't we simply keep the same code as for the CONFIG_RTNETLINK case ?

Thanks in advance for any comment,

Regards,
Willy
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