linux-kernel - Re: [PATCH] rxrpc: struct mutex cannot be used for rxrpc_call::user

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Message-ID: <20191218203946.GL11457@worktop.programming.kicks-ass.net>
Date:   Wed, 18 Dec 2019 21:39:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     David Howells <dhowells@...hat.com>, linux-afs@...ts.infradead.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] rxrpc: struct mutex cannot be used for
 rxrpc_call::user_mutex

On Wed, Dec 18, 2019 at 11:08:33AM -0800, Davidlohr Bueso wrote:
> On Wed, 18 Dec 2019, Peter Zijlstra wrote:
> 
> > On Tue, Dec 17, 2019 at 03:32:00PM +0000, David Howells wrote:
> > > Standard kernel mutexes cannot be used in any way from interrupt or softirq
> > > context, so the user_mutex which manages access to a call cannot be a mutex
> > > since on a new call the mutex must start off locked and be unlocked within
> > > the softirq handler to prevent userspace interfering with a call we're
> > > setting up.
> > > 
> > > 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.
> > 
> > FYI that patch has currently been reverted.
> > 
> > commit c571b72e2b845ca0519670cb7c4b5fe5f56498a5 (tip/locking/urgent, tip/locking-urgent-for-linus)
> 
> Will we ever want to re-add this warning (along with writer rwsems) at some point?

Yes, we can try again for the next cycle.