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Date:   Thu, 26 Oct 2017 08:58:28 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     =?UTF-8?q?Ronald=20Tschal=C3=A4r?= 
        <ronald@...ovation.ch>
Cc:     Marcel Holtmann <marcel@...tmann.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Dean Jenkins <Dean_Jenkins@...tor.com>,
        linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Bluetooth: hci_ldisc: Allow sleeping while proto
 locks are held.

On Wed, Oct 25, 2017 at 10:14:53PM -0700, =?UTF-8?q?Ronald=20Tschal=C3=A4r?= wrote:
> Commit dec2c92880cc5435381d50e3045ef018a762a917 ("Bluetooth: hci_ldisc:
> Use rwlocking to avoid closing proto races") introduced locks in
> hci_ldisc that are held while calling the proto functions. These locks
> are rwlock's, and hence do not allow sleeping while they are held.
> However, the proto functions that hci_bcm registers use mutexes and
> hence need to be able to sleep.
[...]
> We can't replace the mutex in hci_bcm, because there are other calls
> there that might sleep. Therefore this replaces the rwlock's in
> hci_ldisc with rw_semaphore's (which allow sleeping). This is a safer
> approach anyway as it reduces the restrictions on the proto callbacks.
> Also, because acquiring write-lock is very rare compared to acquiring
> the read-lock, the percpu variant of rw_semaphore is used.

The percpu_rw_semaphore is unusual (if fine I guess), it's only used
by cgroups, uprobes and ext4 so far and I was unaware of its existence.

I don't have the hardware to test this but the rationale and patch itself
LGTM, so:

Reviewed-by: Lukas Wunner <lukas@...ner.de>

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