Re: RT and Cascade interrupts

[john cooper]

Trond Myklebust wrote:
> lau den 28.05.2005 Klokka 13:48 (-0400) skreiv john cooper:
> Could you please explain why you think such a scenario is possible? The
> timer functions themselves should never be causing a re-queue, and every
> iteration through the loop in __rpc_execute() should cause any pending
> timer to be killed, as should rpc_release_task().

I'm trying to pinpoint the cause of the RPC timer cascade
structure corruption.  Here is the data I've ascertained thus far:

1.  the failure only has been seen for the timer struct embedded
    in an rpc_task.  The callback function is always rpc_run_timer().

2.  the failure mode is an rpc_task's embedded timer struct being
    queued a second time in the timer cascade without having
    been dequeued from an existing cascade vector.  The failure
    is not immediate but causes a corrupt entry which is later
    encountered in timer cascade code.

3.  the problem occurs if rpc_run_timer() executes in preemptable
    context.  The problem was not encountered when preemption was
    explicitly disabled within rpc_run_timer().

4.  the problem appears related to RPC_TASK_HAS_TIMER not being set
    in rpc_release_task().  Specifically the problem arises when
    !RPC_TASK_HAS_TIMER but timer->base is non-zero.  Modifying
    rpc_delete_timer() to effect the del_singleshot_timer_sync()
    call when (RPC_TASK_HAS_TIMER || timer->base) prevents the
    failure.

5.  (a detail of #2 above) Instrumenting the number of logical
    add/mod/del operations on the timer struct the corruption
    occurs at a point when the tally of the operations:
    (mod == add == del + 1) ie: the timer is has been add/modified
    several times but one count greater than the number of delete
    operations (active in cascade).  The next operation on this
    timer struct is an add/mod operation with the state of the
    timer having been reinitialized in rpc_init_task():init_timer()
    with no intervening delete having been performed.

If it wasn't clear in my prior mail, please disregard the earlier
claim of RPC_TASK_HAS_TIMER replicating the state of timer->base.
From Oleg's earlier mail I see that isn't the case as additional
RPC-specific state is attached to this flag.  The patch as well
should be disregarded.

> That's why we can use del_singleshot_timer_sync() in the first place:
> because there is no recursion, and no re-queueing that can cause races.
> I don't see how either preemption or RT will change that (and if they
> do, then _that_ is the real bug that needs fixing).

During the time I have been hunting this bug I've lost
count of the number of times I've alternatively suspected
either the kernel timer code or net/sunrpc/sched.c usage
of the same.  I still feel it is somehow related to the RPC
code but will need to refine the instrumentation to extract
further information from the failure scenario.

A few questions I'd like to pose:

1.  Can you correlate anything of rpc_run_timer() running in
    preemptive context which could explain the above behavior?

2.  Do you agree that (!RPC_TASK_HAS_TIMER && timer->base)
    is an inconsistent state at the time of
    rpc_release_task():rpc_delete_timer() ?


-john


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