linux-kernel - Re: [PATCH 6/8] blk-mq: remove REQ_ATOM

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Date:   Mon, 8 Jan 2018 23:36:20 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jbacik@...com" <jbacik@...com>, "tj@...nel.org" <tj@...nel.org>,
        "jack@...e.cz" <jack@...e.cz>, "clm@...com" <clm@...com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>
Subject: Re: [PATCH 6/8] blk-mq: remove REQ_ATOM_COMPLETE usages from blk-mq

On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
> After the recent updates to use generation number and state based
> synchronization, blk-mq no longer depends on REQ_ATOM_COMPLETE except
> to avoid firing the same timeout multiple times.
> 
> Remove all REQ_ATOM_COMPLETE usages and use a new rq_flags flag
> RQF_MQ_TIMEOUT_EXPIRED to avoid firing the same timeout multiple
> times.  This removes atomic bitops from hot paths too.
> 
> v2: Removed blk_clear_rq_complete() from blk_mq_rq_timed_out().
> 
> v3: Added RQF_MQ_TIMEOUT_EXPIRED flag.

I think it's unfortunate that this patch spreads the request state over two
struct request members, namely the lower two bits of gstate and the
RQF_MQ_TIMEOUT_EXPIRED flag in req->rq_flags. Please consider to drop the
RQF_MQ_TIMEOUT_EXPIRED flag and to represent the "timeout has been processed"
state as a MQ_RQ_* state in gstate. That will avoid that every state update
has to be reviewed whether or not perhaps an update of the
RQF_MQ_TIMEOUT_EXPIRED flag is missing.

Thanks,

Bart.