linux-kernel - Re: [PATCH] block: paride: on26: Replace mdelay with msleep in on26_test

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Date:   Fri, 26 Jan 2018 17:31:30 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     tim@...erelk.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: paride: on26: Replace mdelay with msleep in
 on26_test_port

On Fri, Jan 26, 2018 at 11:42:25PM +0800, Jia-Ju Bai wrote:
> After checking all possible call chains to on26_test_port() here,
> my tool finds that this function is never called in atomic context,
> namely never in an interrupt handler or holding a spinlock.
> And on26_test_port() is only called by pi_probe_unit() that calls 
> wait_event() through pi_claim(),
> so it indicates that on26_test_port() can call functions that can sleep.
> Thus mdelay can be replaced with msleep to avoid busy wait.

Sigh...  Here's how I would've written it:

"
on26_test_port() is never called from atomic contexts.

It has no direct callers and it is reachable only via ->test_port.
->test_port has only one user:
drivers/block/paride/paride.c:322:              max = pi->proto->test_port(pi);
in pi_probe_unit().  That gets called only from pi_init(), called from
p{d,cd,f,t,g}_detect(), called from module_init stuff, all of the above
without entering atomic contexts along the way.

Despite never getting called from atomic contexts, on26_test_port() contains
mdelay(100), i.e. busy-loops for 0.1s; that's neither nice nor needed, since
msleep() would serve just as well.

Found by [reference to tool]"