linux-kernel - Re: [PATCH 3/5] blktrace: refcount the request

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Message-ID: <20200416052909.GI11244@42.do-not-panic.com>
Date:   Thu, 16 Apr 2020 05:29:09 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Christoph Hellwig <hch@...radead.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,
        mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 3/5] blktrace: refcount the request_queue during ioctl

On Wed, Apr 15, 2020 at 08:43:32PM -0700, Bart Van Assche wrote:
> On 2020-04-15 18:12, Luis Chamberlain wrote:
> > On Wed, Apr 15, 2020 at 07:18:22AM -0700, Bart Van Assche wrote:
> >> blk_get_queue() prevents concurrent freeing of struct request_queue but
> >> does not prevent concurrent blk_cleanup_queue() calls.
> > 
> > Wouldn't concurrent blk_cleanup_queue() calls be a bug? If so should
> > I make it clear that it would be or simply prevent it?
> 
> I think calling blk_cleanup_queue() while the queue refcount > 0 is well
> established behavior. At least the SCSI core triggers that behavior
> since a very long time. I prefer not to change that behavior.

I see. An alternative is to simply check if we already are cleaning up
and if so abort early on the blk_cleanup_queue(). That would allow
re-entrant calls, and just be a no-op to the additional calls. Or is
the re-entrant, two attemps to really do all the work
blk_cleanup_queue() expected functionality already?

> Regarding patch 3/5: how about dropping that patch? If the queue
> refcount can drop to zero while blk_trace_ioctl() is in progress I think
> that should be fixed in the block_device_operations.open callback
> instead of in blk_trace_ioctl().

I'll take a look, thanks!

  Luis