/**
 * lib/minmax.c: windowed min/max tracker
 *
 * Kathleen Nichols' algorithm for tracking the minimum (or maximum)
 * value of a data stream over some fixed time interval.  (E.g.,
 * the minimum RTT over the past five minutes.) It uses constant
 * space and constant time per update yet almost always delivers
 * the same minimum as an implementation that has to keep all the
 * data in the window.
 *
 * The algorithm keeps track of the best, 2nd best & 3rd best min
 * values, maintaining an invariant that the measurement time of
 * the n'th best >= n-1'th best. It also makes sure that the three
 * values are widely separated in the time window since that bounds
 * the worse case error when that data is monotonically increasing
 * over the window.
 *
 * Upon getting a new min, we can forget everything earlier because
 * it has no value - the new min is <= everything else in the window
 * by definition and it's the most recent. So we restart fresh on
 * every new min and overwrites 2nd & 3rd choices. The same property
 * holds for 2nd & 3rd best.
 */
#include <linux/module.h>
#include <linux/win_minmax.h>

/* As time advances, update the 1st, 2nd, and 3rd choices. */
static u32 minmax_subwin_update(struct minmax *m, u32 win,
				const struct minmax_sample *val)
{
	u32 dt = val->t - m->s[0].t;

	if (unlikely(dt > win)) {
		/*
		 * Passed entire window without a new val so make 2nd
		 * choice the new val & 3rd choice the new 2nd choice.
		 * we may have to iterate this since our 2nd choice
		 * may also be outside the window (we checked on entry
		 * that the third choice was in the window).
		 */
		m->s[0] = m->s[1];
		m->s[1] = m->s[2];
		m->s[2] = *val;
		if (unlikely(val->t - m->s[0].t > win)) {
			m->s[0] = m->s[1];
			m->s[1] = m->s[2];
			m->s[2] = *val;
		}
	} else if (unlikely(m->s[1].t == m->s[0].t) && dt > win/4) {
		/*
		 * We've passed a quarter of the window without a new val
		 * so take a 2nd choice from the 2nd quarter of the window.
		 */
		m->s[2] = m->s[1] = *val;
	} else if (unlikely(m->s[2].t == m->s[1].t) && dt > win/2) {
		/*
		 * We've passed half the window without finding a new val
		 * so take a 3rd choice from the last half of the window
		 */
		m->s[2] = *val;
	}
	return m->s[0].v;
}

/* Check if new measurement updates the 1st, 2nd or 3rd choice max. */
u32 minmax_running_max(struct minmax *m, u32 win, u32 t, u32 meas)
{
	struct minmax_sample val = { .t = t, .v = meas };

	if (unlikely(val.v >= m->s[0].v) ||	  /* found new max? */
	    unlikely(val.t - m->s[2].t > win))	  /* nothing left in window? */
		return minmax_reset(m, t, meas);  /* forget earlier samples */

	if (unlikely(val.v >= m->s[1].v))
		m->s[2] = m->s[1] = val;
	else if (unlikely(val.v >= m->s[2].v))
		m->s[2] = val;

	return minmax_subwin_update(m, win, &val);
}
EXPORT_SYMBOL(minmax_running_max);

/* Check if new measurement updates the 1st, 2nd or 3rd choice min. */
u32 minmax_running_min(struct minmax *m, u32 win, u32 t, u32 meas)
{
	struct minmax_sample val = { .t = t, .v = meas };

	if (unlikely(val.v <= m->s[0].v) ||	  /* found new min? */
	    unlikely(val.t - m->s[2].t > win))	  /* nothing left in window? */
		return minmax_reset(m, t, meas);  /* forget earlier samples */

	if (unlikely(val.v <= m->s[1].v))
		m->s[2] = m->s[1] = val;
	else if (unlikely(val.v <= m->s[2].v))
		m->s[2] = val;

	return minmax_subwin_update(m, win, &val);
}
ion value='1'>1</option><option value='2'>2</option><option value='3' selected='selected'>3</option><option value='4'>4</option><option value='5'>5</option><option value='6'>6</option><option value='7'>7</option><option value='8'>8</option><option value='9'>9</option><option value='10'>10</option><option value='15'>15</option><option value='20'>20</option><option value='25'>25</option><option value='30'>30</option><option value='35'>35</option><option value='40'>40</option></select></td></tr><tr><td class='label'>space:</td><td class='ctrl'><select name='ignorews' onchange='this.form.submit();'><option value='0' selected='selected'>include</option><option value='1'>ignore</option></select></td></tr><tr><td class='label'>mode:</td><td class='ctrl'><select name='dt' onchange='this.form.submit();'><option value='0' selected='selected'>unified</option><option value='1'>ssdiff</option><option value='2'>stat only</option></select></td></tr><tr><td/><td class='ctrl'><noscript><input type='submit' value='reload'/></noscript></td></tr></table></form></div><table summary='commit info' class='commit-info'>
<tr><th>author</th><td>Linus Torvalds &lt;torvalds@linux-foundation.org&gt;</td><td class='right'>2017-01-29 13:50:06 -0800</td></tr>
<tr><th>committer</th><td>Linus Torvalds &lt;torvalds@linux-foundation.org&gt;</td><td class='right'>2017-01-29 13:50:06 -0800</td></tr>
<tr><th>commit</th><td colspan='2' class='oid'><a href='/cgit.cgi/linux/net-next.git/commit/net/ipv6/datagram.c?id=39cb2c9a316e77f6dfba96c543e55b6672d5a37e'>39cb2c9a316e77f6dfba96c543e55b6672d5a37e</a> (<a href='/cgit.cgi/linux/net-next.git/patch/net/ipv6/datagram.c?id=39cb2c9a316e77f6dfba96c543e55b6672d5a37e'>patch</a>)</td></tr>
<tr><th>tree</th><td colspan='2' class='oid'><a href='/cgit.cgi/linux/net-next.git/tree/?id=39cb2c9a316e77f6dfba96c543e55b6672d5a37e'>98fe974ee4e20121253de7f61fc8d01bdb3821c1</a> /<a href='/cgit.cgi/linux/net-next.git/tree/net/ipv6/datagram.c?id=39cb2c9a316e77f6dfba96c543e55b6672d5a37e'>net/ipv6/datagram.c</a></td></tr>
<tr><th>parent</th><td colspan='2' class='oid'><a href='/cgit.cgi/linux/net-next.git/commit/net/ipv6/datagram.c?id=2c5d9555d6d937966d79d4c6529a5f7b9206e405'>2c5d9555d6d937966d79d4c6529a5f7b9206e405</a> (<a href='/cgit.cgi/linux/net-next.git/diff/net/ipv6/datagram.c?id=39cb2c9a316e77f6dfba96c543e55b6672d5a37e&amp;id2=2c5d9555d6d937966d79d4c6529a5f7b9206e405'>diff</a>)</td></tr></table>
<div class='commit-subject'>drm/i915: Check for NULL i915_vma in intel_unpin_fb_obj()</div><div class='commit-msg'>I've seen this trigger twice now, where the i915_gem_object_to_ggtt()
call in intel_unpin_fb_obj() returns NULL, resulting in an oops
immediately afterwards as the (inlined) call to i915_vma_unpin_fence()
tries to dereference it.

It seems to be some race condition where the object is going away at
shutdown time, since both times happened when shutting down the X
server.  The call chains were different:

 - VT ioctl(KDSETMODE, KD_TEXT):

    intel_cleanup_plane_fb+0x5b/0xa0 [i915]
    drm_atomic_helper_cleanup_planes+0x6f/0x90 [drm_kms_helper]
    intel_atomic_commit_tail+0x749/0xfe0 [i915]
    intel_atomic_commit+0x3cb/0x4f0 [i915]
    drm_atomic_commit+0x4b/0x50 [drm]
    restore_fbdev_mode+0x14c/0x2a0 [drm_kms_helper]
    drm_fb_helper_restore_fbdev_mode_unlocked+0x34/0x80 [drm_kms_helper]
    drm_fb_helper_set_par+0x2d/0x60 [drm_kms_helper]
    intel_fbdev_set_par+0x18/0x70 [i915]
    fb_set_var+0x236/0x460
    fbcon_blank+0x30f/0x350
    do_unblank_screen+0xd2/0x1a0
    vt_ioctl+0x507/0x12a0
    tty_ioctl+0x355/0xc30
    do_vfs_ioctl+0xa3/0x5e0
    SyS_ioctl+0x79/0x90
    entry_SYSCALL_64_fastpath+0x13/0x94

 - i915 unpin_work workqueue:

    intel_unpin_work_fn+0x58/0x140 [i915]
    process_one_work+0x1f1/0x480
    worker_thread+0x48/0x4d0
    kthread+0x101/0x140

and this patch purely papers over the issue by adding a NULL pointer
check and a WARN_ON_ONCE() to avoid the oops that would then generally
make the machine unresponsive.

Other callers of i915_gem_object_to_ggtt() seem to also check for the
returned pointer being NULL and warn about it, so this clearly has
happened before in other places.

[ Reported it originally to the i915 developers on Jan 8, applying the
  ugly workaround on my own now after triggering the problem for the
  second time with no feedback.

  This is likely to be the same bug reported as

     https://bugs.freedesktop.org/show_bug.cgi?id=98829
     https://bugs.freedesktop.org/show_bug.cgi?id=99134

  which has a patch for the underlying problem, but it hasn't gotten to
  me, so I'm applying the workaround. ]

Cc: Daniel Vetter &lt;daniel.vetter@intel.com&gt;
Cc: Jani Nikula &lt;jani.nikula@linux.intel.com&gt;
Cc: Ville Syrjälä &lt;ville.syrjala@linux.intel.com&gt;
Cc: Chris Wilson &lt;chris@chris-wilson.co.uk&gt;
Cc: Maarten Lankhorst &lt;maarten.lankhorst@linux.intel.com&gt;
Cc: Tvrtko Ursulin &lt;tvrtko.ursulin@intel.com&gt;
Cc: Imre Deak &lt;imre.deak@intel.com&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
</div><div class='diffstat-header'><a href='/cgit.cgi/linux/net-next.git/diff/?id=39cb2c9a316e77f6dfba96c543e55b6672d5a37e'>Diffstat</a> (limited to 'net/ipv6/datagram.c')</div><table summary='diffstat' class='diffstat'>