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Subject[PATCH] timer: Fix possible issues with non serialized timer_pending( )
Date
When stress testing ARC Linux from 3.9-rc3, we've hit a serialization
issue when mod_timer() races with itself. This is on a FPGA board and
kernel .config among others has !SMP and !PREEMPT_COUNT.

The issue happens in mod_timer( ) because timer_pending( ) based early
exit check is NOT done inside the timer base spinlock - as a networking
optimization.

The value used in there, timer->entry.next is also used further in call
chain (all inlines though) for actual list manipulation. However if the
register containing this pointer remains live across the spinlock (in a
UP setup with !PREEMPT_COUNT there's nothing forcing gcc to reload) then
a stale value of next pointer causes incorrect list manipulation,
observed with following sequence in our tests.

(0). tv1[x] <----> t1 <---> t2
(1). mod_timer(t1) interrupted after it calls timer_pending()
(2). mod_timer(t2) completes
(3). mod_timer(t1) resumes but messes up the list.
(4). __runt_timers( ) uses bogus timer_list entry / crashes in
timer->function

The simplest fix is to NOT rely on spinlock based compiler barrier but
add an explicit one in timer_pending()

FWIW, the relevant ARCompact disassembly of mod_timer which clearly
shows the issue due to register reuse is:

mod_timer:
push_s blink
mov_s r13,r0 # timer, timer

...
###### timer_pending( )
ld_s r3,[r13] # <------ <variable>.entry.next LOADED
brne r3, 0, @.L163

.L163:
....
###### spin_lock_irq( )
lr r5, [status32] # flags
bic r4, r5, 6 # temp, flags,
and.f 0, r5, 6 # flags,
flag.nz r4

###### detach_if_pending( ) begins

tst_s r3,r3 <--------------
# timer_pending( ) checks timer->entry.next
# r3 is NOT reloaded by gcc, using stale value
beq.d @.L169
mov.eq r0,0

# detach_timer( ): __list_del( )

ld r4,[r13,4] # <variable>.entry.prev, D.31439
st r4,[r3,4] # <variable>.prev, D.31439
st r3,[r4] # <variable>.next, D.30246

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Reported-by: Christian Ruppert <christian.ruppert@abilis.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Christian Ruppert <christian.ruppert@abilis.com>
Cc: Pierrick Hascoet <pierrick.hascoet@abilis.com>
Cc: linux-kernel@vger.kernel.org
---
include/linux/timer.h | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 8c5a197..1537104 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -168,7 +168,16 @@ static inline void init_timer_on_stack_key(struct timer_list *timer,
*/
static inline int timer_pending(const struct timer_list * timer)
{
- return timer->entry.next != NULL;
+ int pending = timer->entry.next != NULL;
+
+ /*
+ * The check above enables timer fast path - early exit.
+ * However most of the call sites are not protected by timer->base
+ * spinlock. If the caller (say mod_timer) races with itself, it
+ * can use the stale "next" pointer. See commit log for details.
+ */
+ barrier();
+ return pending;
}

extern void add_timer_on(struct timer_list *timer, int cpu);
--
1.7.10.4


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