linux-kernel - Re: [RFC PATCH 5/9] ntsync: Introduce NTSYNC_IOC_WAIT

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Date: Thu, 25 Jan 2024 12:30:28 -0600
From: Elizabeth Figura <zfigura@...eweavers.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
 Arnd Bergmann <arnd@...db.de>
Cc: wine-devel@...ehq.org,
 André Almeida <andrealmeid@...lia.com>,
 Wolfram Sang <wsa@...nel.org>, Arkadiusz Hiler <ahiler@...eweavers.com>,
 Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 5/9] ntsync: Introduce NTSYNC_IOC_WAIT_ANY.

On Thursday, 25 January 2024 11:02:26 CST Arnd Bergmann wrote:
> On Wed, Jan 24, 2024, at 23:28, Elizabeth Figura wrote:
> > On Wednesday, 24 January 2024 13:52:52 CST Arnd Bergmann wrote:
> >> On Wed, Jan 24, 2024, at 19:02, Elizabeth Figura wrote:
> >> > That'd be nicer in general. I think there was some documentation that
> >> > advised using timespec64 for new ioctl interfaces but it may have been
> >> > outdated or misread.
> >> 
> >> It's probably something I wrote. It depends a bit on
> >> whether you have an absolute or relative timeout. If
> >> the timeout is relative to the current time as I understand
> >> it is here, a 64-bit number seems more logical to me.
> >> 
> >> For absolute times, I would usually use a __kernel_timespec,
> >> especially if it's CLOCK_REALTIME. In this case you would
> >> also need to specify the time domain.
> > 
> > Currently the interface does pass it as an absolute time, with the
> > domain implicitly being MONOTONIC. This particular choice comes from
> > process/botching-up-ioctls.rst, which is admittedly focused around GPU
> > ioctls, but the rationale of having easily restartable ioctls applies
> > here too.
> 
> Ok, I was thinking of Documentation/driver-api/ioctl.rst, which
> has similar recommendations.
> 
> > (E.g. Wine does play games with signals, so we do want to be able to
> > interrupt arbitrary waits with EINTR. The "usual" fast path for ntsync
> > waits won't hit that, but we want to have it work.)
> > 
> > On the other hand, if we can pass the timeout as relative, and write it
> > back on exit like ppoll() does [assuming that's not proscribed], that
> > would presumably be slightly better for performance.
> 
> I've seen arguments go either way between absolute and relative
> times, just pick whatever works best for you here.
> 
> > When writing the patch I just picked the recommended option, and didn't
> > bother doing any micro-optimizations afterward.
> > 
> > What's the rationale for using timespec for absolute or written-back
> > timeouts, instead of dealing in ns directly? I'm afraid it's not
> > obvious to me.
> 
> There is no hard rule either way, I mainly didn't like the
> indirect pointer to the timespec that you have here. For
> traditional unix-style interfaces, a timespec with CLOCK_REALTIME
> times usually makes sense since that is what user space is
> already using elsewhere, but you probably don't need to
> worry about that. In theory, the single u64 CLOCK_REALTIME
> nanoseconds have the problem of no longer working after year
> 2262, but with CLOCK_MONOTONIC that is not a concern anyway.
> 
> Between embedding a __u64 nanosecond value and embedding
> a __kernel_timespec, I would pick whichever avoids converting
> a __u64 back into a timespec, as that is an expensive
> operation at least on 32-bit code.

Makes sense. I'll probably switch to using a relative and written-back u64 
then, thanks!