Re: [PATCH] rxrpc: Mutexes are unusable from softirq context, so use rwsem instead

From: Peter Zijlstra
Date: Tue Dec 10 2019 - 14:30:20 EST

On Tue, Dec 10, 2019 at 08:10:09PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 10, 2019 at 05:32:58PM +0000, David Howells wrote:
> > rxrpc_call::user_mutex is of type struct mutex, but it's required to start
> > off locked on an incoming call as it is being set up in softirq context to
> > prevent sendmsg and recvmsg interfering with it until it is ready. It is
> > then unlocked in rxrpc_input_packet() to make the call live.
> >
> > Unfortunately, commit a0855d24fc22d49cdc25664fb224caee16998683
> > ("locking/mutex: Complain upon mutex API misuse in IRQ contexts") causes
> > big warnings to be splashed in dmesg for each a new call that comes in from
> > the server.
> >
> > It *seems* like it should be okay, since the accept path trylocks the mutex
> > when no one else can see it and drops the mutex before it leaves softirq
> > context.
> >
> > Fix this by switching to using an rw_semaphore instead as that is permitted
> > to be used in softirq context.
> This really has the very same problem. It just avoids the WARN. We do PI
> boosting for rwsem write side identical to what we do for mutexes.
> I would rather we revert David's patch for now and more carefully
> consider what to do about this.

To clarify (I only just reliazed David is a bit ambiguous here), take
this patch out for now:

a0855d24fc22 ("locking/mutex: Complain upon mutex API misuse in IRQ contexts")

The RXRPC code has been there for a while... and like I wrote, both
mutex and rwsem have the exact same issue, the rwsem code just doesn't
have a WARN on it.