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    SubjectRe: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.
    On Wed, 29 Oct 2008, Miklos Szeredi wrote:

    > Actually I was thinking of an rw-semaphore, not a mutex. But yeah
    > that still has scalability problems. But it could be done with custom
    > locking primitives, optimized for this case:
    > suspend_disable();
    > /* driver stuff */
    > suspend_enable();

    Yes, it could be done. And the overhead could be minimized by using
    per-CPU variables. It would still be an awful lot of work, and easy to
    get wrong.

    > > The problem with unrestricted freezing shows up when you freeze tasks
    > > that hold a mutex or other sort of lock. If this mutex is needed later
    > > on for suspending a device then the suspend will hang, because a frozen
    > > task can't release any mutexes.
    > I did a random sampling of ->suspend() callbacks, and they don't seem
    > to be taking mutexes. Does that happen at all?

    It does, particularly among drivers that do runtime PM, which is
    becoming more and more important.

    Besides, suspend has to synchronize with I/O somehow. Right now that
    is handled by making suspend wait until no tasks are doing I/O (because
    they are all frozen). If you allow tasks to be frozen at more or less
    arbitrary times, while holding arbitrary locks, then you may end up
    freezing a task that's in the middle of I/O. That should certainly
    block the suspend (not to mention messing up the I/O operation).

    > Did anybody ever try modifying the freezer for suspend (not
    > hibernate), so that it allows tasks not in running state to freeze?
    > If not, I think that's an experiment worth doing.

    What happens if the reason the task isn't running is because it's
    waiting for I/O to complete? I just don't think this can be made to

    Alan Stern

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      Last update: 2008-10-29 16:31    [W:0.043 / U:23.736 seconds]
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