xtime_lock vs update_process_times

Commit d3d74453c34f8fd87674a8cf5b8a327c68f22e99 ("hrtimer: fixup the
HRTIMER_CB_IRQSAFE_NO_SOFTIRQ fallback") broke several archs, and since
only Russell bothered to merge the fix, and Greg to ACK his arch, I'm
sending this for merger.

I have confirmation that the Alpha bit results in a booting kernel.
That leaves: blackfin, frv, sh and sparc untested.

The deadlock in question was found by Russell:

  IRQ handle
    -> timer_tick() - xtime seqlock held for write
      -> update_process_times()
        -> run_local_timers()
          -> hrtimer_run_queues()
            -> hrtimer_get_softirq_time() - tries to get a read lock

Now, Thomas assures me the fix is trivial, only do_timer() needs to be
done under the xtime_lock, and update_process_times() can savely be
removed from under it.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Greg Ungerer <gerg@uclinux.org>
CC: Richard Henderson <rth@twiddle.net>
CC: Bryan Wu <bryan.wu@analog.com>
CC: David Howells <dhowells@redhat.com>
CC: Paul Mundt <lethal@linux-sh.org>
CC: William Irwin <wli@holomorphy.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 7 deletions

diff --git a/arch/alpha/kernel/time.c b/arch/alpha/kernel/time.c
index 1dd50d07693c..75480cab0893 100644
--- a/arch/alpha/kernel/time.c
+++ b/arch/alpha/kernel/time.c
@@ -119,13 +119,8 @@ irqreturn_t timer_interrupt(int irq, void *dev)
 	state.partial_tick = delta & ((1UL << FIX_SHIFT) - 1); 
 	nticks = delta >> FIX_SHIFT;
 
-	while (nticks > 0) {
-		do_timer(1);
-#ifndef CONFIG_SMP
-		update_process_times(user_mode(get_irq_regs()));
-#endif
-		nticks--;
-	}
+	if (nticks)
+		do_timer(nticks);
 
 	/*
 	 * If we have an externally synchronized Linux clock, then update
@@ -141,6 +136,12 @@ irqreturn_t timer_interrupt(int irq, void *dev)
 	}
 
 	write_sequnlock(&xtime_lock);
+
+#ifndef CONFIG_SMP
+	while (nticks--)
+		update_process_times(user_mode(get_irq_regs()));
+#endif
+
 	return IRQ_HANDLED;
 }