linux-kernel - Re: futex timeout not working? (bisected)

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Date:	Fri, 16 Oct 2015 12:54:51 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Stefan Liebler <stli@...ux.vnet.ibm.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: futex timeout not working? (bisected)

On Fri, Oct 16, 2015 at 12:35:23PM +0200, Thomas Gleixner wrote:
> On Fri, 16 Oct 2015, Heiko Carstens wrote:
> > On Fri, Oct 16, 2015 at 12:21:51PM +0200, Thomas Gleixner wrote:
> > > Heiko,
> > > 
> > > On Thu, 15 Oct 2015, Heiko Carstens wrote:
> > > >   /* We wait for half a second.  */
> > > >   ts.tv_nsec += 500000000;
> > > >   if (ts.tv_nsec >= 1000000000)
> > > >     {
> > > >       ++ts.tv_sec;
> > > >       ts.tv_nsec -= 1000000000;
> > > >     }
> > > > 
> > > >   ret = syscall (__NR_futex, &futex_word, op, expected, ts, NULL, FUTEX_BITSET_MATCH_ANY);
> > > 
> > > If you actually hand in a pointer to 'ts' then the program works as
> > > expected even with the commit in question applied.
> > 
> > Yes, sorry about that! I was just about to write that this should have
> > been &ts on x86. Since x86 seems to pass also structures >8 bytes by
> > value.
> > 
> > However on s390 it is passed by reference. So even it I change ts to &ts,
> > it will hang as I described.
> > 
> > So you are saying this works on x86 and it must be some s390 specific bug?
> 
> Looks like. I have no idea why that would break on s390. Did you try
> to revert the commit on top of tree?

Reverting the patch isn't easy due to all the dependencies the code has in
the meantime. However the following code snippet does logically revert the
patch and it works again:

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 3739ac6aa473..c97dd1cc9bd5 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1953,12 +1953,10 @@ ktime_t ktime_get_update_offsets_now(unsigned int *cwsseq, ktime_t *offs_real,
 		nsecs = timekeeping_get_ns(&tk->tkr_mono);
 		base = ktime_add_ns(base, nsecs);
 
-		if (*cwsseq != tk->clock_was_set_seq) {
-			*cwsseq = tk->clock_was_set_seq;
-			*offs_real = tk->offs_real;
-			*offs_boot = tk->offs_boot;
-			*offs_tai = tk->offs_tai;
-		}
+		*cwsseq = tk->clock_was_set_seq;
+		*offs_real = tk->offs_real;
+		*offs_boot = tk->offs_boot;
+		*offs_tai = tk->offs_tai;
 
 		/* Handle leapsecond insertion adjustments */
 		if (unlikely(base.tv64 >= tk->next_leap_ktime.tv64))

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