Re: [CHECKER] a couple potential deadlocks in 2.4.5-ac8

Rusty Russell
2001-06-10T11:52:36Z

In message <19317.992115181@redhat.com> you write:
>
> torvalds@transmeta.com said:
> > Good point. Spinlocks (with the exception of read-read locks, of
> > course) and semaphores will deadlock on recursive use, while the BKL
> > has this "process usage counter" recursion protection.
>
> Obtaining a read lock twice can deadlock too, can't it?
>
> A B
> read_lock()
> write_lock()
> ...sleeps...
> read_lock()
> ...sleeps...
>
> Or do we not make new readers sleep if there's a writer waiting?

We can never[1] make new readers sleep if there's a writer waiting, as
Linus guaranteed that an IRQ handler which only ever grabs a read lock
means the rest of the code doesn't need to block interrupts on its
read locks (see Documentation/spinlock.txt IIRC).

Also, netfilter will break (brlocks inherit this property from
their spinlocks constituents).

Rusty.
[1] Well, we could, but we'd have to do a special "same CPU?" check,
which would suck badly.
--
Premature optmztion is rt of all evl. --DK