linux-kernel - Re: [PATCH 2/2] tracing/events: block: dev_t via driver core for plug and unplug events

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Message-ID: <8624e2bec853f0bb195966ab54bc42bf15eda0ec.camel@wdc.com>
Date:   Thu, 19 Apr 2018 20:56:47 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "maier@...ux.ibm.com" <maier@...ux.ibm.com>,
        "osandov@...ndov.com" <osandov@...ndov.com>
CC:     "lizefan@...wei.com" <lizefan@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "lizf@...fujitsu.com" <lizf@...fujitsu.com>
Subject: Re: [PATCH 2/2] tracing/events: block: dev_t via driver core for plug
 and unplug events

On Thu, 2018-04-19 at 12:24 -0700, Omar Sandoval wrote:
> We quiesce and freeze the queue before tearing it down, so it won't be
> NULL while we're still doing I/O. Cc'ing Bart in case I'm lying to you,
> though ;)

blk_cleanup_queue() waits until all I/O requests have finished. Since the
block layer tracing code is triggered from inside the code that processes a
request it is safe to access the request pointer from inside the tracing code.
But I think the question was about the parent of the request queue kobj
instead of about the request queue pointer, the device structure that is
embedded in struct gendisk? I think that parent can disappear at any time.
Most block drivers call del_gendisk() before they call blk_cleanup_queue().
Unless I'm overlooking something I think the request queue will need to
obtain a reference to the gendisk device from inside blk_register_queue()
and drop that reference from inside blk_cleanup_queue() to make it safe for
the tracing code to access struct gendisk.

Bart.