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

[Peter Zijlstra]

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.