Re: [PATCH v2] locking/rtmutex: Always use trylock in rt_mutex_trylock()

[Peter Zijlstra]

On Wed, Oct 02, 2024 at 03:01:08PM -0400, Waiman Long wrote:
> One reason to use a trylock is to avoid a ABBA deadlock in case we need
> to use a locking sequence that is not in the expected locking order. That
> requires the use of trylock all the ways in the abnormal locking
> sequence. Unfortunately, this is not the case for rt_mutex_trylock() as
> it uses a raw_spin_lock_irqsave() to acquire the lock->wait_lock.

This does not explain anything. lock->wait_lock only serializes the lock
internal state and should be fine to be taken like this.

> There are just a few rt_mutex_trylock() call sites in the stock kernel.
> For PREEMPT_RT kernel, however, all the spin_trylock() calls become
> rt_mutex_trylock(). There are a few hundreds of them. So it will be a lot
> easier to trigger a circular locking lockdep splat like the following.
> 
> [   63.695668] -> #0 (&lock->wait_lock){-...}-{2:2}:
> [   63.695674]        check_prev_add+0x1bd/0x1310
> [   63.695678]        validate_chain+0x6cf/0x7c0
> [   63.695682]        __lock_acquire+0x879/0xf40
> [   63.695686]        lock_acquire.part.0+0xfa/0x2d0
> [   63.695690]        _raw_spin_lock_irqsave+0x46/0x90
> [   63.695695]        rt_mutex_slowtrylock+0x3f/0xb0
> [   63.695699]        rt_spin_trylock+0x13/0xc0
> [   63.695702]        rmqueue_pcplist+0x5b/0x180
> [   63.695705]        rmqueue+0xb01/0x1040
>      :
> [   63.695840] other info that might help us debug this:
> [   63.695840]
> [   63.695842] Chain exists of:
> [   63.695842]   &lock->wait_lock --> &p->pi_lock --> &rq->__lock
> [   63.695842]
> [   63.695850]  Possible unsafe locking scenario:
> [   63.695850]
> [   63.695851]        CPU0                    CPU1
> [   63.695852]        ----                    ----
> [   63.695854]   lock(&rq->__lock);
> [   63.695857]                                lock(&p->pi_lock);
> [   63.695861]                                lock(&rq->__lock);
> [   63.695864]   lock(&lock->wait_lock);
> [   63.695868]
> [   63.695868]  *** DEADLOCK ***
> 

This is still useless crap. Please properly describe the locking problem
without truncated lockdep crud.