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

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Message-ID: <20191218202801.wokf6hcvbafmjnkd@linux-p48b>
Date:   Wed, 18 Dec 2019 12:28:01 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
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, 18 Dec 2019, 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?
>
>It seems that having it actually prompts things getting fixed, as opposed to
>just sitting there forever borken (at least in -rt).

Hmm so fyi __crash_kexec() is another one, but can be called in hard-irq, and
it's extremely obvious that the trylock+unlock occurs in the same context.

It would be nice to automate this...

Thanks,
Davidlohr