On Thu, 21 Dec 2006 10:41:44 +0100
"Sorin Manolache" <[email protected]> wrote:
> The Linux Device Drivers book says that a spin_lock should not be
> shared between a process and an interrupt handler. The explanation is
> that the process may hold the lock, an interrupt occurs, the interrupt
> handler spins on the lock held by the process and the system freezes.
> Why should it freeze? Isn't it possible for the interrupt handler to
> re-enable interrupts as its first thing, then to spin at the lock, the
> timer interrupt to preempt the interrupt handler and to relinquish
> control to the process which in turn will finish its critical section
> and release the lock, making way for the interrupt handler to
> continue.
Iterrupt handlers are executend in the process context (on top of the
process that they interrupted).
So, if you have a proccess A that does:
Usual Kernel Code Interrupt Handler
...
spin_lock(my_lock);
...
-------interrupt-----> ...
spin_lock(my_lock); // deadlock!
...
<------ back ---------
----
spin_unlock(my_lock);
See?
If the interrupt comes in when process A is running and holding the
lock PREEMPTION can't do anything.
--
Paolo Ornati
Linux 2.6.20-rc1-g99f5e971 on x86_64
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