Re: MM deadlock [was: Re: arca-vm-8...]

[Linus Torvalds <torvalds@transmeta.com>]

On Sat, 9 Jan 1999, Savochkin Andrey Vladimirovich wrote:
> 
> I've found an another deadlock.

Yes. This is a case I knew about, and that Alan already mentioned. Trying
to write from a shared mapping has a path that can take the write
semaphore twice.

This one is a whole lot harder to fix - the previous one needed only a
simple extra flag, this one is truly nasty.

The cleanest solution I can think of is actually to allow semaphores to be
recursive. I can do that with minimal overhead (just one extra instruction
in the non-contention case), so it's not too bad, and I've wanted to do it
for certain other things, but it's still a nasty piece of code to mess
around with. 

Oh, well. I don't think I have much choice. Making the swap-out routines
refuse to touch an inode that is busy is a sure way to allow people to
let bad users lock down infinite amounts of memory.

		Linus

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