[BUG] etnaviv: broken timeouts
Russell King - ARM Linux
linux at armlinux.org.uk
Wed Oct 25 09:19:07 UTC 2017
On Mon, Oct 23, 2017 at 04:21:57PM +0100, Russell King - ARM Linux wrote:
> On Mon, Oct 23, 2017 at 03:57:39PM +0100, Russell King - ARM Linux wrote:
> > On Thu, Aug 31, 2017 at 02:49:27PM +0200, Lucas Stach wrote:
> > > Hi Russell,
> > >
> > > Am Donnerstag, den 31.08.2017, 12:18 +0100 schrieb Russell King - ARM
> > > Linux:
> > > > I've just stumbled on a bug related to the way we handle fence
> > > > timeouts.
> > > >
> > > > For DRM_ETNAVIV_WAIT_FENCE, we have:
> > > >
> > > > struct drm_etnaviv_wait_fence {
> > > > __u32 pipe; /* in */
> > > > __u32 fence; /* in */
> > > > __u32 flags; /* in, mask of ETNA_WAIT_x */
> > > > __u32 pad;
> > > > struct drm_etnaviv_timespec timeout; /* in */
> > > > };
> > > >
> > > > where timeout is:
> > > >
> > > > struct drm_etnaviv_timespec {
> > > > __s64 tv_sec; /* seconds */
> > > > __s64 tv_nsec; /* nanoseconds */
> > > > };
> > > >
> > > > The timeout is with respect to the monotonic clock. If the timeout is
> > > > specified far enough in the future, eg:
> > > >
> > > > 9088652.2192296410 now 4793684.242296410
> > > >
> > > > then rather than waiting, the function returns almost immediately with
> > > > ETIMEDOUT. The requested timeout is equivalent to (uint32_t)~0
> > > > milliseconds.
> > > >
> > > > In the kernel, we take the drm_etnaviv_timespec, and stick it into a
> > > > struct timespec via the TS() macro. This gets passed to
> > > > etnaviv_gpu_wait_fence_interruptible(), which uses
> > > > etnaviv_timeout_to_jiffies() to convert to jiffies. I suspect that
> > > > the conversion to jiffies in timespec_to_jiffies() results in a
> > > > jiffy value that time_after() believes to be before the current time,
> > > > resulting in ultimately a zero jiffy timeout.
> > > >
> > > > Merely stracing the X server, or adding a fprintf() is enough to
> > > > avoid the problem.
> > > >
> > > > If you hit this problem, you'll see "fence finish failed" in the Xorg
> > > > log.
> > > >
> > > > I think doing the time_after() dance after converting to jiffies is
> > > > wrong: if we're going to have an API that accepts absolute time, then
> > > > we should handle times that are beyond the ability for us to schedule
> > > > the wait correctly.
> > > >
> > > > It looks like other APIs that take a timespec or timeval (eg, ppoll(),
> > > > select(), pselect()) convert the timespec to a ktime value, which
> > > > limits to KTIME_MAX (see time*_to_ktime() and ktime_set()), which is
> > > > a much nicer behaviour than that which the etnaviv DRM driver is
> > > > currently giving us.
> > > >
> > > Are you going to provide a patch for this, or should I take a look at
> > > fixing this?
> >
> > Okay, the good news is that it's finally triggered again, and I now have
> > debugging in both the kernel and X server to work out what's going on.
> >
> > There's a whole host of things wrong here.
> >
> > Here's the debug from one event:
> >
> > X: timed out: to=8868123.581486415 start=4573156.286486415 end=4573156.286602102
> >
> > What this is telling me is that CLOCK_MONOTONIC was reporting a time of
> > 4573156.286486415s, which we used as the basis for calculating the
> > timeout. The requested timeout was VIV_WAIT_INDEFINITE (32-bit ~0 ms).
> > Hence, the calculated timeout was 8868123.581486415s. However, the
> > kernel reported timeout, and CLOCK_MONOTONIC reported 4573156.286602102s
> > immediately after - barely 200us later.
> >
> > From the kernel side, we have:
> >
> > [4573105.970932] timed out: timeout=8868123.581486415 jiffies=1143214071
> > remaining=0 fence=1056827139:1056827138
> >
> > The timeout target value corresponds. This is a 250Hz kernel, so the
> > jiffies value corresponds with 1143214071/250s, or 4572856.284s.
> >
> > The way etnaviv_timeout_to_jiffies() is currently written, it first
> > tries to convert the timeout to a number of jiffies. The first step
> > in that is to convert the timeout to a timespec64, and then to jiffies
> > using timespec64_to_jiffies().
> >
> > The maximum value that timespec64_to_jiffies() will convert is 4294966s
> > on my kernel, gathered by reading the disassembly:
> >
> > 00000268 <timespec64_to_jiffies>:
> > ...
> > 274: e1c020d0 ldrd r2, [r0]
> > 278: e3a01000 mov r1, #0
> > ...
> > 280: e3080936 movw r0, #35126 ; 0x8936
> > 284: e3400041 movt r0, #65 ; 0x41
> > 288: e1510003 cmp r1, r3
> > 28c: 01500002 cmpeq r0, r2
> > 290: 2a000002 bcs 2a0 <timespec64_to_jiffies+0x38>
> > 294: e30f0fb6 movw r0, #65462 ; 0xffb6
> > 298: e3430fff movt r0, #16383 ; 0x3fff
> > 29c: e89dadf0 ldm sp, {r4, r5, r6, r7, r8, sl, fp, sp, pc}
> >
> > and this returns a maximum of 0x3fffffb6 jiffies, or 1073741750.
> > Since the requested timeout is past the maximum number of seconds,
> > we return this as "timeout_jiffies".
> >
> > start_jiffies is 1143214071, and we can see immediately that
> > time_after(1143214071, 1073741750) is going to be true.
> >
> > This causes remaining_jiffies to be zero, as reported in the debug
> > printk as "remaining".
> >
> > What's more is that we can see that CLOCK_MONOTONIC reported about
> > 4573156.286486415s, and the jiffies value was 300s in the past,
> > reporting 4572856.284s.
> >
> > So, the calculations here are totally bogus.
> >
> > I propose the following (build tested only) to fix this, but testing
> > it is going to be very problematical - it's taken 52 days to reproduce
> > the original problem. So, the only sensible way to make sure that
> > this is correct is to get many eyes on it and review it thoroughly.
> >
> > There's a multitude of different time functions, I just picked here
> > what looked the most sensible, so this may not be the best solution
> > here - maybe Thomas can advise.
> >
> > The other thing to consider is what happens if the userspace
> > CLOCK_MONOTONIC value wraps - what should the driver behaviour be?
> > Converting the userspace absolute timeout figure to a timespec64
> > means that it'll be seen to be in the past, and this problem will
> > rear its head again. Maybe we're better to sticking to the
> > normal timespec functions so that _our_ idea of monotonic time
> > wraps in unison, so the requested timeout continues to work -
> > although I'm not convinced timespec*() works across wraps.
> >
> > Other things... it looks to me as if timespec64_to_jiffies() isn't
> > doing the right thing - why, on a HZ=250 kernel, does it limit
> > jiffies to (~0 / 1000)s and not (~0 / 250)s? (I'm grateful that
> > it did, because if it allowed even twice that figure, I'd be
> > looking at a 104 day reproduction time for this issue.)
> >
> > In any case, what this IMHO shows is that dealing with timeouts in
> > terms of tv_sec/tv_nsec values is frought with problems - and if
> > every driver is needing to open code some kind of absolute timeout
> > to jiffies conversion to use in the kernel's standard wait functions,
> > I can see many could fall into similar traps. I think either we
> > need to discourage the use of tv_sec/tv_nsec absolute timeout APIs
> > or we need some way to help drivers get this calculation correct.
>
> Note: although drm_etnaviv_timespec supports 64-bit timespec, there's
> no way to set it to a valid 64-bit timespec, since clock_gettime() in
> userspace for a 32-bit arch won't return 64-bit time. So, while it
> looks like it would be sane to convert etnaviv's timespec pointers to
> timespec64, that solves nothing (in fact, I believe it makes it worse.)
>
> I do think the patch below should not be using the 64-bit timespec
> until the last conversion to jiffies.
Here's the "normal" timespec version, which I think is preferable to
the timespec64 version:
diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.h b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
index e41f38667c1c..a435f2c7309a 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_drv.h
+++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
@@ -143,19 +143,29 @@ static inline bool fence_after_eq(u32 a, u32 b)
return (s32)(a - b) >= 0;
}
+/*
+ * Etnaviv timeouts are specified wrt CLOCK_MONOTONIC, not jiffies.
+ * We need to calculate the timeout in terms of number of jiffies
+ * between the specified timeout and the current CLOCK_MONOTONIC time.
+ * Note: clock_gettime() is 32-bit on 32-bit arch. Using 64-bit
+ * timespec math here just means that when a wrap occurs, the
+ * specified timeout goes into the past and we can't request a
+ * timeout in the future: IOW, the code breaks.
+ */
static inline unsigned long etnaviv_timeout_to_jiffies(
const struct timespec *timeout)
{
- unsigned long timeout_jiffies = timespec_to_jiffies(timeout);
- unsigned long start_jiffies = jiffies;
- unsigned long remaining_jiffies;
+ struct timespec ts;
+
+ ktime_get_ts(&ts);
+
+ /* timeouts before "now" have already expired */
+ if (timespec_compare(timeout, &ts) <= 0)
+ return 0;
- if (time_after(start_jiffies, timeout_jiffies))
- remaining_jiffies = 0;
- else
- remaining_jiffies = timeout_jiffies - start_jiffies;
+ ts = timespec_sub(*timeout, ts);
- return remaining_jiffies;
+ return timespec_to_jiffies(&ts);
}
#endif /* __ETNAVIV_DRV_H__ */
--
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