linux-kernel - RE: Clarification required about select vs wake

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Date:	Wed, 14 Mar 2007 11:38:18 -0700
From:	"Ravinandan Arakali \(rarakali\)" <rarakali@...co.com>
To:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
Cc:	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: RE: Clarification required about select vs wake_up race condition

Dmitry,
Thanks for the explanation. That would mean that if
process is not put to sleep, it will go back to
top of for() loop in do_select(), set state back
to TASK_INTERRUPTIBLE and re-check for incoming data.

Ravi 

-----Original Message-----
From: Dmitry Adamushko [mailto:dmitry.adamushko@...il.com] 
Sent: Wednesday, March 14, 2007 10:08 AM
To: Ravinandan Arakali (rarakali)
Cc: Linux Kernel
Subject: Re: Clarification required about select vs wake_up race
condition

On 12/03/07, Ravinandan Arakali (rarakali) <rarakali@...co.com> wrote:
> Hi,
> I am facing following problem and was wondering if somebody could help

> me out.
> Our char driver(pretty much like all other char drivers) does a
> poll_wait()
> and returns status depending on whether data is available to be read.
> Even though some data is available to be read(verified using one of 
> our internal commands), the select() never wakes up, inspite of any 
> no. of messages sent.
>
> To understand this, I was looking at the code of select vs 
> wake_up_interruptible().
> I feel I am misunderstanding some part of the kernel code but will be 
> glad if somebody can point it out.
>
> My understanding:
> The do_select() sets the state of task to TASK_INTERRUPTIBLE and calls

> the driver's poll entry point. In our poll(), let's say immediately 
> after we determine that there's nothing to be read, some data arrives 
> causing a wake_up_interruptible() on another CPU.
> The wake up happens in the context of process sending the data. Since 
> the receiving process was already added to the list of listeners, from

> looking at the code of try_to_wake_up(), it looks like it can set the 
> state of the receiving process to TASK_RUNNING(I don't see any lock 
> preventing this). After this happens, the receiving process goes to 
> sleep (because of schedule_timeout called by do_select) but state is 
> still set to TASK_RUNNING.

No, it's not going to sleep then.

The effect of schedule() being called with current->state ==
TASK_RUNNING is a re-scheduling to another task with a higher prio (if
any) or just getting back (iow, the task doesn't lose a cpu). For both
cases, the task is on the runqueue.

/ Look how/when deactivate_task() is called in schedule() /

Maybe there is a race in your code between (1) how you check "data is
available" in poll and (2) a part that sets this fact (data is
available)...


--
Best regards,
Dmitry Adamushko
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