linux-kernel - Re: [PATCH] wait_for_completion

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Message-ID: <20141231103711.GA30533@opentech.at>
Date:	Wed, 31 Dec 2014 11:37:11 +0100
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Cc:	Inaky Perez-Gonzalez <inaky.perez-gonzalez@...el.com>,
	linux-wimax@...el.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] wait_for_completion_timeout does not return < 0

On Wed, 31 Dec 2014, Sudip Mukherjee wrote:

> On Wed, Dec 31, 2014 at 04:04:03AM -0500, Nicholas Mc Guire wrote:
> > This is only removing the comment which is misleading as
> > wait_for_completion_timeout does not return < 0 thus there
> > never is anything to be passed on.
> 
> a small doubt - 
> i am seeing that:
> unsigned long wait_for_completion_timeout() is calling
> long wait_for_common()  which is again calling 
> long __wait_for_common which is again calling
> long do_wait_for_common()
> 
> now the return value from do_wait_for_common can be -ERESTARTSYS,
> so then what happens when wait_for_completion_timeout return this -ERESTARTSYS as an unsigned value ?
> it becomes a positive value, and ultimately ctx.result (which is 0) is returned.
> so then are we just ignoring the error value from do_wait_for_common() ?
>
 
ESTARTSYS only can be returned if state matches in do_wait_for_common
but wait_for_completion_timemout passes TASK_UNINTERRUPTIBLE
so signal_pending_state will return 0 and thus it will never
return -ERESTARTSYS.

my understanding of the callchain is:
wait_for_completion_timemout which is uninterruptible
  -> wait_for_common(...TASK_UNINTERRUPTIBLE)
    -> __wait_for_common(...TASK_UNINTERRUPTIBLE)
      -> do_wait_for_common(...TASK_UNINTERRUPTIBLE)
        -> signal_pending_state(TASK_UNINTERRUPTIBLE...)

static inline int signal_pending_state(long state, struct task_struct *p)
{
        if (!(state & (TASK_INTERRUPTIBLE | TASK_WAKEKILL)))
                return 0;

so wait_for_completion_timemout should return >=0 only

thx!
hofrat

> > 
> > patch is against linux-next 3.19.0-rc1 -next-20141226
> > 
> > Signed-off-by: Nicholas Mc Guire <der.herr@...r.at>
> > ---
> >  drivers/net/wimax/i2400m/driver.c |    1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/drivers/net/wimax/i2400m/driver.c b/drivers/net/wimax/i2400m/driver.c
> > index 9c78090..0a6384e 100644
> > --- a/drivers/net/wimax/i2400m/driver.c
> > +++ b/drivers/net/wimax/i2400m/driver.c
> > @@ -197,7 +197,6 @@ int i2400m_op_reset(struct wimax_dev *wimax_dev)
> >  		result = -ETIMEDOUT;
> >  	else if (result > 0)
> >  		result = ctx.result;
> > -	/* if result < 0, pass it on */
> >  	mutex_lock(&i2400m->init_mutex);
> >  	i2400m->reset_ctx = NULL;
> >  	mutex_unlock(&i2400m->init_mutex);
> > -- 
> > 1.7.10.4
> > 
> > --
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