linux-kernel - Re: [PATCH] scsi: ips.c: use 64-bit time types

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Message-ID: <2518561.k9qJZyWstd@wuerfel>
Date:	Thu, 09 Oct 2014 16:29:52 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Ebru Akagunduz <ebru.akagunduz@...il.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	opw-kernel@...glegroups.com
Subject: Re: [PATCH] scsi: ips.c: use 64-bit time types

On Thursday 09 October 2014 06:40:26 James Bottomley wrote:
> On Wed, 2014-10-08 at 22:58 +0200, Arnd Bergmann wrote:
> > On Wednesday 08 October 2014 13:44:55 James Bottomley wrote:
> > > > diff --git a/drivers/scsi/ips.h b/drivers/scsi/ips.h
> > > > index 45b9566..ff2a0b3 100644
> > > > --- a/drivers/scsi/ips.h
> > > > +++ b/drivers/scsi/ips.h
> > > > @@ -1054,7 +1054,7 @@ typedef struct ips_ha {
> > > >     uint8_t            active;
> > > >     int                ioctl_reset;        /* IOCTL Requested Reset Flag */
> > > >     uint16_t           reset_count;        /* number of resets           */
> > > > -   time_t             last_ffdc;          /* last time we sent ffdc info*/
> > > > +   time64_t             last_ffdc;          /* last time we sent ffdc info*/
> > > >     uint8_t            slot_num;           /* PCI Slot Number            */
> > > >     int                ioctl_len;          /* size of ioctl buffer       */
> > > >     dma_addr_t         ioctl_busaddr;      /* dma address of ioctl buffer*/
> > > 
> > > This is completely pointless, isn't it?  All the ips driver cares about
> > > is that we send a FFDC time update every eight hours or so, so we can
> > > happily truncate the number of seconds to 32 bits for that calculation
> > > just keep the variable at 32 bits and do a time_after thing for the
> > > comparison.
> > 
> > Good point. The same has come up in a few other places, so I wonder if we
> > should introduce a proper way to do it that doesn't involve time_t.
> 
> We have, it's jiffies ... that's why I'm slightly non-plussed that this
> driver is using gettimeofday for something like this ... it was clearly
> a review failure when we put it in.

Actually there is more to it, as I just found upon reading the code
again (I had noticed it before when I first looked at the driver but
then forgotten about it):

ips_fix_ffdc_time() needs the correct current wall-clock time, no overflow
allowed, to stick the year/month/day/hour/minute/second value into
the ffdc command.

My comment to Ebru about ktime_get_ts64 for monotonic time was unfortunately
completely wrong, since that would break whatever timekeeping it is
in the hardware that wants the correct year/month/day/hour/minute/second
values.

> or are you thinking we need a time_t_time_before doing for time_t what
> we do for jiffies?

The part I'm interested in is getting rid of any mention of time_t,
timespec and timeval in the kernel by replacing each use with something
that is known to be y2038-safe. Using jiffies correctly would solve
a number of them, but is not sufficient for this driver because of the
ffdc command.

We could use jiffies to test whether we need to send ffdc but then
we still need to read the correct time.

> > While the current code works, we will have to audit 2000 other locations
> > in which time_t/timespec/timeval are used in the kernel, so we are going
> > to need some form of annotation to make sure we don't get everyone to
> > look at the driver again just to come to the same conclusion after working
> > on a patch first.
> > 
> > > However, what the code *should* be doing is using jiffies and
> > > time_before/after since the interval is so tiny rather than a
> > > do_gettimeofday() call in the fast path.
> > 
> > Yes, this would probably be best for this particular driver, it also
> > means we end up with a monotonic clock source rather than a wall-clock.
> 
> Right, and it's a 32 bit read instead of a system call every time the
> thing dispatches a command ... to be honest the overhead of 64 bit
> arithmetic is peanuts to making a syscall in the fast path.

It's not a system call, all we need is a simple function call that reads
tk->xtime_sec. We can use get_seconds() today, but it returns an
'unsigned long', so that won't be enough on 32-bit architectures.

It's still slightly more expensive to do the function call and use a 64-bit
number on a 32-bit CPU, but it's not on the scale of doing a system call
here. You can probably judge best if it's worth the increase in complexity
to use jiffies for determining whether to send the update and then
use get_seconds64 (or similar) to read the wall-clock time, or whether
always using get_seconds64 would be good enough.

	Arnd
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