linux-kernel - [PATCH] Timekeeping: Fix dead lock in update_wall

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Message-ID: <1268736225.5075.10.camel@eight.analog.com>
Date:	Tue, 16 Mar 2010 18:43:45 +0800
From:	sonic zhang <sonic.adi@...il.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] Timekeeping: Fix dead lock in update_wall_time by correct
	shift convertion.

update_wall_time() runs into dead lock after kernel traps into kgdb and exits
per user's request some seconds layer. This is root caused to be wrong
calculation of maxshift in update_wall_time().

The shift in update_wall_time() and logarithmic_accumulation() is
clock shift. In order to generate ntp_error and maxshift correctly,
shift convertion between clock and ntp should be done properly.


Signed-off-by: Sonic Zhang <sonic.zhang@...log.com>
---
 kernel/time/timekeeping.c |   20 ++++++++++++++------
 1 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 1673637..5b47c9e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -770,10 +770,14 @@ static cycle_t logarithmic_accumulation(cycle_t
offset, int shift)
                raw_time.tv_sec++;
        }
 
-       /* Accumulate error between NTP and clock interval */
-       timekeeper.ntp_error += tick_length << shift;
-       timekeeper.ntp_error -= timekeeper.xtime_interval <<
-                               (timekeeper.ntp_error_shift + shift);
+       /*
+        * Accumulate error between NTP and clock interval.
+        * Paramter shift is clock shift. It should minus shift
+        * conversion between clock and ntp to generate ntp shift.
+        */
+       timekeeper.ntp_error += tick_length <<
+                               (shift - timekeeper.ntp_error_shift);
+       timekeeper.ntp_error -= timekeeper.xtime_interval << shift;
 
        return offset;
 }
@@ -813,8 +817,12 @@ void update_wall_time(void)
         */
        shift = ilog2(offset) - ilog2(timekeeper.cycle_interval);
        shift = max(0, shift);
-       /* Bound shift to one less then what overflows tick_length */
-       maxshift = (8*sizeof(tick_length) - (ilog2(tick_length)+1)) - 1;
+       /*
+        * Bound shift to one less then what overflows tick_length.
+        * Should plus shift conversion between clock and ntp.
+        */
+       maxshift = (8*sizeof(tick_length) - (ilog2(tick_length)+1)) - 1
+                       + timekeeper.ntp_error_shift;
        shift = min(shift, maxshift);
        while (offset >= timekeeper.cycle_interval) {
                offset = logarithmic_accumulation(offset, shift);
-- 
1.6.0




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