linux-kernel - Re: [PATCH v7 8/8] block: create the request_queue debugfs

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Message-ID: <02112994-4cd7-c749-6bd7-66a772593c90@acm.org>
Date:   Sat, 20 Jun 2020 11:07:43 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org
Cc:     mhocko@...e.com, yukuai3@...wei.com, martin.petersen@...cle.com,
        jejb@...ux.ibm.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 8/8] block: create the request_queue debugfs_dir on
 registration

On 2020-06-19 13:47, Luis Chamberlain wrote:
> We were only creating the request_queue debugfs_dir only
> for make_request block drivers (multiqueue), but never for
> request-based block drivers. We did this as we were only
> creating non-blktrace additional debugfs files on that directory
> for make_request drivers. However, since blktrace *always* creates
> that directory anyway, we special-case the use of that directory
> on blktrace. Other than this being an eye-sore, this exposes
> request-based block drivers to the same debugfs fragile
> race that used to exist with make_request block drivers
> where if we start adding files onto that directory we can later
> run a race with a double removal of dentries on the directory
> if we don't deal with this carefully on blktrace.
> 
> Instead, just simplify things by always creating the request_queue
> debugfs_dir on request_queue registration. Rename the mutex also to
> reflect the fact that this is used outside of the blktrace context.

There are two changes in this patch: a bug fix and a rename of a mutex.
I don't like it to see two changes in a single patch.

Additionally, is the new mutex name really better than the old name? The
proper way to use mutexes is to use mutexes to protect data instead of
code. Where is the documentation that mentions which member variable(s)
of which data structures are protected by the mutex formerly called
blk_trace_mutex? Since the new name makes it even less clear which data
is protected by this mutex, is the new name really better than the old name?

Thanks,

Bart.