linux-kernel - Do not deprecate binary semaphore or do allow mutex in software interrupt contexts

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Message-ID: <36061.50670.qm@web52002.mail.re2.yahoo.com>
Date:	Tue, 11 Sep 2007 09:20:23 -0700 (PDT)
From:	Matti Linnanvuori <mattilinnanvuori@...oo.com>
To:	linux-kernel@...r.kernel.org
Subject: Do not deprecate binary semaphore or do allow mutex in software interrupt contexts

 Arjan van de Ven:
> what do you do if the trylock fails?

Just do not read the status variable now but modify the timer to run later.

> to be honest, the scenario describe really smells of broken locking, in
> fact it really sounds like it wants to use spinlocks instead 

No, I don't think it is broken.
Spinlocks can be used, but I don't see them being obviously better in all cases.
If access takes a long time, it is better to sleep during it.
And if you sleep, you might just end up creating a new mutex implementation with a spinlock.

Alan Cox:
> For polling and timer based code its often simpler to do
>
>    del_timer_sync(&my_timer);
>    FrobStuff
>    add_timer(&my_timer);
>
> especially if "FrobStuff" is likely to change when you next need to poll.

In the scenario I presented, the timer modifies itself to run later.
Therefore, simply calling del_timer_sync is not enough but you have to set an atomic variable to prevent the timer from adding itself again.
Again, you end up creating a new mutex implementation, which is not good.

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