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Date:   Sat, 2 Mar 2019 01:54:16 +0900
From:   "Tokunori Ikegami" <ikegami.t@...il.com>
To:     "'Boris Brezillon'" <boris.brezillon@...labora.com>
Cc:     "'Tokunori Ikegami'" <ikegami.t@...il.com>,
        <keescook@...omium.org>, <bbrezillon@...nel.org>, <richard@....at>,
        <linux-kernel@...r.kernel.org>, <marek.vasut@...il.com>,
        <linux-mtd@...ts.infradead.org>, <computersforpeace@...il.com>,
        <dwmw2@...radead.org>, "'liujian \(CE\)'" <liujian56@...wei.com>,
        <vigneshr@...com>
Subject: RE: [PATCH v3] cfi: fix deadloop in cfi_cmdset_0002.c do_write_buffer

Hi Boris-san,

> -----Original Message-----
> From: linux-mtd [mailto:linux-mtd-bounces@...ts.infradead.org] On Behalf
> Of Boris Brezillon
> Sent: Saturday, March 2, 2019 1:07 AM
> To: Tokunori Ikegami
> Cc: 'Tokunori Ikegami'; keescook@...omium.org; bbrezillon@...nel.org;
> ikegami@...ied-telesis.co.jp; richard@....at;
> linux-kernel@...r.kernel.org; marek.vasut@...il.com;
> linux-mtd@...ts.infradead.org; computersforpeace@...il.com;
> dwmw2@...radead.org; 'liujian (CE)'; vigneshr@...com
> Subject: Re: [PATCH v3] cfi: fix deadloop in cfi_cmdset_0002.c
> do_write_buffer
> 
> Hi Ikegami,
> 
> On Fri, 1 Mar 2019 23:51:16 +0900
> "Tokunori Ikegami" <ikegami_to@...oo.co.jp> wrote:
> 
> > > Except this version no longer does what Vignesh suggested. See how you
> > > no longer test if chip_good() is true if time_after() returns true.
> So,
> > > imagine the thread entering this function is preempted just after the
> > > first chip_good() test, and resumed a few ms later. time_after() will
> > > return true, but chip_good() might also return true, and you ignore
> it.
> >
> > I think that the following 3 versions will be worked for time_after()
> as a same result and follow the Vignesh-san suggestion.
> 
> Let me show you how they are different:
> 
> >
> > 1. Original Vignesh-san suggestion
> >
> > 	if (chip_good(map, adr, datum)) {
> > 		xip_enable(map, chip, adr);
> > 		goto op_done;
> > 	}
> 
> --> thread preempted here
> ==> chip_good() test becomes valid here
> --> thread resumed here, but timeout has expired
> 
> >
> > 	if (time_after(jiffies, timeo)) {
> 
> you enter this branch
> 
> > 		/* Test chip_good() if TRUE incorrectly again so write
> failure by time_after() can be avoided. */
> > 		if (chip_good(map, adr, datum)) {
> 
> chip_good() returns true
> 
> > 			xip_enable(map, chip, adr);
> > 			goto op_done;
> > 		}
> > 		break;
> > 	}
> >
> > 2. Liujian-san v3 patch
> >
> > 	/* Test chip_good() if FALSE correctly so write failure by
> time_after() can be avoided. */
> 
> --> thread preempted here
> ==> chip_good() test becomes valid here
> --> thread resumed here, but timeout has expired
> 
> > 	if (time_after(jiffies, timeo) && !chip_good(map, adr))
> 
> You do not enter this branch because the chip_good() test is done once
> more in case of timeout.
> 
> > 		break;
> >
> > 	if (chip_good(map, adr, datum)) {
> > 		xip_enable(map, chip, adr);
> > 		goto op_done;
> > 	}
> >
> > 3. My idea
> >
> > 	/* Save current jiffies value before chip_good() to avoid write
> failure by time_after() without testing chip_good() again. */
> > 	unsigned long now = jiffies;
> >
> > 	if (chip_good(map, adr, datum)) {
> > 		xip_enable(map, chip, adr);
> > 		goto op_done;
> > 	}
> >
> 
> --> thread preempted here
> ==> chip_good() test becomes valid here
> --> thread resumed here, but timeout has expired
> 
> > 	if (time_after(now, timeo))
> 
> You do enter this branch, and erroneously report a failure.

I do not think that it is not entered here since the value timeo is compare
with the saved value now before the chip_bood() by time_after().

> 
> > 		break;
> >
> 
> See now why your version is not correct?
> 
> Regards,
> 
> Boris
> 
> ______________________________________________________
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