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>

2 files changed, 4 insertions, 5 deletions

diff --git a/arch/sparc/kernel/pcic.c b/arch/sparc/kernel/pcic.c
index 4cd5d7818dc6..a6a6f9823370 100644
--- a/arch/sparc/kernel/pcic.c
+++ b/arch/sparc/kernel/pcic.c
@@ -713,10 +713,10 @@ static irqreturn_t pcic_timer_handler (int irq, void *h)
         write_seqlock(&xtime_lock);     /* Dummy, to show that we remember */
         pcic_clear_clock_irq();
         do_timer(1);
+        write_sequnlock(&xtime_lock);
 #ifndef CONFIG_SMP
         update_process_times(user_mode(get_irq_regs()));
 #endif
-        write_sequnlock(&xtime_lock);
         return IRQ_HANDLED;
 }
 
diff --git a/arch/sparc/kernel/time.c b/arch/sparc/kernel/time.c
index 00b393c3a4a0..cfaf22c05bc4 100644
--- a/arch/sparc/kernel/time.c
+++ b/arch/sparc/kernel/time.c
@@ -128,10 +128,6 @@ irqreturn_t timer_interrupt(int irq, void *dev_id)
         clear_clock_irq();
 
         do_timer(1);
-#ifndef CONFIG_SMP
-        update_process_times(user_mode(get_irq_regs()));
-#endif
-
 
         /* Determine when to update the Mostek clock. */
         if (ntp_synced() &&
@@ -145,6 +141,9 @@ irqreturn_t timer_interrupt(int irq, void *dev_id)
         }
         write_sequnlock(&xtime_lock);
 
+#ifndef CONFIG_SMP
+        update_process_times(user_mode(get_irq_regs()));
+#endif
         return IRQ_HANDLED;
 }