[PATCH] Preserve task state in reentrant calls to ___wait_event

Chris Bainbridge
2015-11-06T20:44:08Z

In the ACPI SBS initialisation, a reentrant call to wait_event_timeout()
causes an intermittent boot stall of several minutes usually following
the "Switching to clocksource tsc" message. This stall is caused by:

1. drivers/acpi/sbshc.c wait_transaction_complete() calls
wait_event_timeout():

if (wait_event_timeout(hc->wait, smb_check_done(hc),
msecs_to_jiffies(timeout)))

2. ___wait_event sets task state to uninterruptible

3. ___wait_event calls the "condition" smb_check_done()

4. smb_check_done (sbshc.c) calls through to ec_read() in
drivers/acpi/ec.c

5. ec_guard() is reached which calls wait_event_timeout()

if (wait_event_timeout(ec->wait,
ec_transaction_completed(ec),
guard))

ie. wait_event_timeout() is being called again inside evaluation of
the previous wait_event_timeout() condition

5. The EC IRQ handler calls wake_up() and wakes up the sleeping task in
ec_guard()

6. The task is now in state running even though the wait "condition" is
still being evaluated

7. The "condition" check returns false so ___wait_event calls
schedule_timeout()

8. Since the task state is running, the scheduler immediately schedules
it again

9. This loop repeats for around 250 seconds event though the original
wait_event_timeout was only 1000ms.

This happens because each the call to schedule_timeout() usually
returns immediately, taking less than 1ms, so the jiffies timeout
counter is not decremented. The task is now stuck in a running
state, and so is highly likely to get rescheduled immediately, which
takes less than a jiffy.

The root problem is that wait_event_timeout() does not preserve the task
state when called by tasks that are not running. We fix this by
preserving and restoring the task state in ___wait_event().

Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
---
I am assuming here that wait_event_timeout() is supposed to support reentrant
calls. If not, perhaps it should BUG_ON when called with a non-running task
state, and the SBS HC / ACPI EC code needs to be fixed to stop doing this. If
reentrant calls are supposed to work, then this patch will preserve the task
state (there may be a more appropriate way to support reentrant calls than this
exact patch, suggestions/alternatives are welcome, but this does work).
---
include/linux/wait.h | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 1e1bf9f..a847cf8 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -209,11 +209,12 @@ wait_queue_head_t *bit_waitqueue(void *, int);
* otherwise.
*/

-#define ___wait_event(wq, condition, state, exclusive, ret, cmd) \
+#define ___wait_event(wq, condition, nstate, exclusive, ret, cmd) \
({ \
__label__ __out; \
wait_queue_t __wait; \
long __ret = ret; /* explicit shadow */ \
+ long ostate = current->state; \
\
INIT_LIST_HEAD(&__wait.task_list); \
if (exclusive) \
@@ -222,16 +223,16 @@ wait_queue_head_t *bit_waitqueue(void *, int);
__wait.flags = 0; \
\
for (;;) { \
- long __int = prepare_to_wait_event(&wq, &__wait, state);\
+ long __int = prepare_to_wait_event(&wq, &__wait, nstate);\
\
if (condition) \
break; \
\
- if (___wait_is_interruptible(state) && __int) { \
+ if (___wait_is_interruptible(nstate) && __int) { \
__ret = __int; \
if (exclusive) { \
abort_exclusive_wait(&wq, &__wait, \
- state, NULL); \
+ nstate, NULL); \
goto __out; \
} \
break; \
@@ -240,6 +241,7 @@ wait_queue_head_t *bit_waitqueue(void *, int);
cmd; \
} \
finish_wait(&wq, &__wait); \
+ set_current_state(ostate); \
__out: __ret; \
})

--
2.1.4