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).
> 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..).
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;
}
All you care about there is bh->state, _not_
common->thread_wakeup_needed.
That said, I cannot spot an obvious fail, but the code can certainly use
help.
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