Messages in this thread | | | Date | Wed, 14 Mar 2007 18:08:02 +0100 | From | "Dmitry Adamushko" <> | Subject | Re: Clarification required about select vs wake_up race condition |
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On 12/03/07, Ravinandan Arakali (rarakali) <rarakali@cisco.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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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