Hi,
Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> On Sat, Sep 03, 2016 at 09:58:09AM +0300, Felipe Balbi wrote:
>
>> > What arch are you seeing this on?
>>
>> x86. Skylake to be exact.
>
> So it _cannot_ be the thing Alan mentioned. By the simple fact that
> spin_lock() is a full barrier on x86 (every LOCK prefixed instruction
> is).
I still have this working even after 15 hours of runtime on a test case
that was failing consistently within few minutes. At a minimum smp_mb()
has some side effect which is hiding the actual problem.
>> The following change survived through the night:
>>
>> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
>> index 8f3659b65f53..d31581dd5ce5 100644
>> --- a/drivers/usb/gadget/function/f_mass_storage.c
>> +++ b/drivers/usb/gadget/function/f_mass_storage.c
>> @@ -395,7 +395,7 @@ static int fsg_set_halt(struct fsg_dev *fsg, struct usb_ep *ep)
>> /* Caller must hold fsg->lock */
>> static void wakeup_thread(struct fsg_common *common)
>> {
>> - smp_wmb(); /* ensure the write of bh->state is complete */
>> + smp_mb(); /* ensure the write of bh->state is complete */
>> /* Tell the main thread that something has happened */
>> common->thread_wakeup_needed = 1;
>> if (common->thread_task)
>> @@ -626,7 +626,7 @@ static int sleep_thread(struct fsg_common *common, bool can_freeze)
>> }
>> __set_current_state(TASK_RUNNING);
>> common->thread_wakeup_needed = 0;
>> - smp_rmb(); /* ensure the latest bh->state is visible */
>> + smp_mb(); /* ensure the latest bh->state is visible */
>> return rc;
>> }
>
> Sorry, but that is horrible code. A barrier cannot ensure writes are
> 'complete', at best they can ensure order between writes (or reads
> etc..).
not arguing ;-)
> Also, looking at that thing, that common->thread_wakeup_needed variable
> is 100% redundant. All sleep_thread() invocations are inside a loop of
> sorts and basically wait for other conditions to become true.
>
> For example:
>
> while (bh->state != BUF_STATE_EMPTY) {
> rc = sleep_thread(common, false);
> if (rc)
> return rc;
> }
right
> All you care about there is bh->state, _not_
> common->thread_wakeup_needed.
>
> That said, I cannot spot an obvious fail,
okay, but a fail does exist. Any hints on what extra information I could
capture to help figuring this one out?
> but the code can certainly use help.
Sure, that can be done for v4.9 (if I have time) or v4.10 merge
window. Meanwhile, we're trying to find a minimal fix for the -rc which
can also be backported to stable, right?
--
balbi
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