В Mon, 29 Dec 2014 21:03:24 +0100
Hans Petter Selasky <h...@selasky.org> пишет:

> Hi,
> I recently came across a class of errors which lead me into 
> investigating the "kern/kern_timeout.c" and its subsystem. From what
> I can see new features like the SMP awareness has been "added"
> instead of fully "integrated". When going into the cornercases I've
> uncovered that the internal callout statemachine can sometimes report
> wrong values via its callout_active() and callout_pending() bits to
> its clients, which in turn can make the clients behave badly. I
> further did an investigation on how the safety of callout migration
> between CPU's is maintained. When I looked into the code and found
> stuff like "volatile" and "while()" loops to figure which CPU a
> callout belongs I understood that such logic completely undermines
> the cleverness found in the turnstiles of mutexes and decided to go
> through all of the logic inside "kern_timeout.c". Also static code
> analysis is harder when we don't use the basic mutexes and condition
> variables available in the kernel.
> First of all we need to make some driving rules for everyone:
> 1) A new feature called direct callbacks which execute the timer 
> callbacks from the fast interrupt handler was added. All these
> callbacks _must_ be associated with a regular spinlocks, to maintain
> a safe callout_drain(). Else they should only be executed on CPU0.
> 2) All Giant locked callbacks should only execute on CPU0 to avoid 
> congestion.
> 3) Callbacks using read-only locks for its callback should also only 
> execute on CPU0 to avoid multiple instances pending for completion on 
> multiple CPU's, because read-only locks can be entered multiple
> times. From what I can see, there are currently no consumers of this
> feature in the kernel.

panic: spin lock held too long
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