From: Bart Van Assche <Bart.VanAssche@wdc.com>
To: "tom.leiming@gmail.com" <tom.leiming@gmail.com>
Cc: "hch@lst.de" <hch@lst.de>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
"jthumshirn@suse.de" <jthumshirn@suse.de>,
"stern@rowland.harvard.edu" <stern@rowland.harvard.edu>,
"axboe@kernel.dk" <axboe@kernel.dk>,
"ming.lei@redhat.com" <ming.lei@redhat.com>
Subject: Re: [PATCH 2/3] block, scsi: Rework runtime power management
Date: Wed, 18 Jul 2018 15:45:15 +0000 [thread overview]
Message-ID: <60ce50f281bf41483b53246d509697f6d95360f8.camel@wdc.com> (raw)
In-Reply-To: <CACVXFVPP6bY9YijDSyQg9OOR4ge9LyR+naWNDeLGOS=WFL_7CA@mail.gmail.com>
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On Wed, 2018-07-18 at 20:16 +-0800, Ming Lei wrote:
+AD4- On Wed, Jul 18, 2018 at 7:49 AM, Bart Van Assche +ADw-bart.vanassche+=
AEA-wdc.com+AD4- wrote:
+AD4- +AD4- +AEAAQA- -3801,8 +-3778,11 +AEAAQA- int blk+AF8-pre+AF8-runtime=
+AF8-suspend(struct request+AF8-queue +ACo-q)
+AD4- +AD4- if (+ACE-q-+AD4-dev)
+AD4- +AD4- return ret+ADs-
+AD4- +AD4-=20
+AD4- +AD4- +- blk+AF8-set+AF8-preempt+AF8-only(q)+ADs-
+AD4- +AD4- +- blk+AF8-freeze+AF8-queue+AF8-start(q)+ADs-
+AD4- +AD4- +-
+AD4- +AD4- spin+AF8-lock+AF8-irq(q-+AD4-queue+AF8-lock)+ADs-
+AD4- +AD4- - if (q-+AD4-nr+AF8-pending) +AHs-
+AD4- +AD4- +- if (+ACE-percpu+AF8-ref+AF8-is+AF8-zero(+ACY-q-+AD4-q+=
AF8-usage+AF8-counter)) +AHs-
+AD4-=20
+AD4- This way can't work reliably because the percpu ref isn't in atomic m=
ode
+AD4- yet after blk+AF8-freeze+AF8-queue+AF8-start() returns, then percpu+A=
F8-ref+AF8-is+AF8-zero() won't
+AD4- see accurate value of the counter, finally the device may be put down=
before
+AD4- in-flight requests are completed by hardware.
Hello Ming,
The blk+AF8-freeze+AF8-queue+AF8-start() implementation is as follows:
void blk+AF8-freeze+AF8-queue+AF8-start(struct request+AF8-queue +ACo-q)
+AHs-
int freeze+AF8-depth+ADs-
freeze+AF8-depth +AD0- atomic+AF8-inc+AF8-return(+ACY-q-+AD4-mq+AF8-freeze=
+AF8-depth)+ADs-
if (freeze+AF8-depth +AD0APQ- 1) +AHs-
percpu+AF8-ref+AF8-kill(+ACY-q-+AD4-q+AF8-usage+AF8-counter)+ADs-
if (q-+AD4-mq+AF8-ops)
blk+AF8-mq+AF8-run+AF8-hw+AF8-queues(q, false)+ADs-
+AH0-
+AH0-
>From the documentation header in include/linux/percpu-refcount.h above
percpu+AF8-ref+AF8-kill():
+ACo- Switches +AEA-ref into atomic mode before gathering up the percpu co=
unters
+ACo- and dropping the initial ref.
In other words, I think that after blk+AF8-freeze+AF8-queue+AF8-start() ret=
urns that it is
guaranteed that q-+AD4-q+AF8-usage+AF8-counter is in atomic mode. However, =
we may need to
serialize concurrent blk+AF8-freeze+AF8-queue+AF8-start() calls to guarante=
e that this is
always the case if multiple threads call blk+AF8-freeze+AF8-queue+AF8-start=
() concurrently.
Bart.=
next prev parent reply other threads:[~2018-07-18 15:45 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-17 23:49 [PATCH 0/3] blk-mq: Enable runtime power management Bart Van Assche
2018-07-17 23:49 ` [PATCH 1/3] block: Fix a comment in a header file Bart Van Assche
2018-07-18 6:47 ` Johannes Thumshirn
2018-07-17 23:49 ` [PATCH 2/3] block, scsi: Rework runtime power management Bart Van Assche
2018-07-18 12:16 ` Ming Lei
2018-07-18 15:45 ` Bart Van Assche [this message]
2018-07-18 22:45 ` Ming Lei
2018-07-19 15:54 ` Bart Van Assche
2018-07-19 23:26 ` Ming Lei
2018-07-17 23:49 ` [PATCH 3/3] blk-mq: Enable support for " Bart Van Assche
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