linux-kernel - Re: [PATCH] Revert "locking/mutex: Complain upon mutex API misuse in IRQ contexts"

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Date:   Tue, 10 Dec 2019 22:32:12 +0000
From:   David Howells <dhowells@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     dhowells@...hat.com, peterz@...radead.org,
        linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        will@...nel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] Revert "locking/mutex: Complain upon mutex API misuse in IRQ contexts"

Davidlohr Bueso <dave@...olabs.net> wrote:

> This ended up causing some noise in places such as rxrpc running in softirq.
> 
> The warning is misleading in this case as the mutex trylock and unlock
> operations are done within the same context; and therefore we need not
> worry about the PI-boosting issues that comes along with no single-owner
> lock guarantees.
> 
> While we don't want to support this in mutexes, there is no way out of
> this yet; so lets get rid of the WARNs for now, as it is only fair to
> code that has historically relied on non-preemptible softirq guarantees.
> In addition, changing the lock type is also unviable: exclusive rwsems
> have the same issue (just not the WARN_ON) and counting semaphores
> would introduce a performance hit as mutexes are a lot more optimized.
> 
> This reverts commit 5d4ebaa87329ef226e74e52c80ac1c62e4948987.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>

Tested-by: David Howells <dhowells@...hat.com>