Date: Tue, 9 Jun 2015 13:55:26 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Sebastian Huber <sebastian.huber@embedded-brains.de> Cc: freebsd-hackers@freebsd.org, phk@phk.freebsd.dk Subject: Re: [PATCH v3] timecounters: Fix timehand generation read/write Message-ID: <20150609105526.GK2499@kib.kiev.ua> In-Reply-To: <1433838715-22850-1-git-send-email-sebastian.huber@embedded-brains.de> References: <1433331966-27548-1-git-send-email-sebastian.huber@embedded-brains.de> <1433838715-22850-1-git-send-email-sebastian.huber@embedded-brains.de>
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On Tue, Jun 09, 2015 at 10:31:55AM +0200, Sebastian Huber wrote:
> The compiler is free to re-order load/store instructions to non-volatile
> variables around a load/store of a volatile variable. So the volatile
> generation counter is insufficent. In addition tests on a Freescale
> T4240 platform with 24 PowerPC processors showed that real memory
> barriers are required. Compiler memory barriers are not enough.
This looks fine. The only detail I changed is that I do not see a reason
to use atomics or barriers on UP machines. See the fragment at the end
of the message for the exact diff.
>
> For the test the timehand count was reduced to one with 10000
> tc_windup() calls per second. The timehand memory location was adjusted
> so that the th_generation field was on its own cache line.
You mean, that the 'th_generation for its own cache line' was done for
testing only ?
> ---
>
> v2: Don't use tc_getgen() in tc_windup() since in the only writer there is no
> need for a read memory barrier.
>
> v3: Use atomic load/store instead of explicit memory barriers.
>
If you do not have any comments, I will commit this version.
diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index 9dca0e8..fabfe03 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -189,6 +190,28 @@ tc_delta(struct timehands *th)
tc->tc_counter_mask);
}
+static u_int
+tc_getgen(struct timehands *th)
+{
+
+#ifdef SMP
+ return (atomic_load_acq_int(&th->th_generation));
+#else
+ return (th->th_generation);
+#endif
+}
+
+static void
+tc_setgen(struct timehands *th, u_int newgen)
+{
+
+#ifdef SMP
+ atomic_store_rel_int(&th->th_generation, newgen);
+#else
+ th->th_generation = newgen;
+#endif
+}
+
/*
* Functions for reading the time. We have to loop until we are sure that
* the timehands that we operated on was not updated under our feet. See
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