Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available | Date | Mon, 19 Mar 2012 12:24:59 +0100 |
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On Monday, March 19, 2012, Stephen Boyd wrote: > On 3/18/2012 5:01 AM, Rafael J. Wysocki wrote: > > On Sunday, March 18, 2012, Stephen Boyd wrote: > > > >> Ok. I like where nowait() is going in the other part of the thread but > >> I'm still confused about when request_firmware() is correct to use. It > >> seems that the function is inherently racy with freezing. Does every > >> user of request_firmware() need to synchronize with freezing? > >> > >> For example, if one CPU is in the middle of a driver probe that makes a > >> request_firmware() call and another CPU is starting to suspend we will > >> have a race between usermodehelpers being disabled and the > >> request_firmware() call acquiring the usermodehelper rwsem. If the > >> suspending CPU wins the race it will disable usermodehelpers and the > >> request_firmware() call will return -EBUSY and warn. > > Yes, it will. > > That sounds wrong then, no? Why don't we have request_firmware() do a > read_lock on the usermodehelpers sem and then have suspend do a write > lock, disable usermodehelpers, wait for any users to finish, freeze > processes and then unlock the write lock? That way we don't hit a case > where request_firmware() races with suspend, and we don't have to change > the warning or conditional.
So, you're postulating that the freezing of tasks be done under umhelper_sem write-locked, right?
That would lead to freezing failures if a user space task waited in request_firmware() for umhelper_sem to become available for read-locking and unfortunately we don't have an interruptible variant of down_read().
However, we may catch request_firmware() and try to freeze the task calling it instead. I'll try to prototype something along these lines later today (on top of the three "firmware_class" patches I posted yesterday).
Thanks, Rafael
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