Re: Deadlock on the mm->mmap_sem

[Arjan van de Ven <arjanv@redhat.com>]

Andrea Arcangeli wrote:
> 
> On Tue, Sep 18, 2001 at 09:31:40AM +0200, Manfred Spraul wrote:
> > From: "Andrea Arcangeli" <andrea@suse.de>
> > > > The mmap semaphore is a read-write semaphore, and it _is_
> > permissible to
> > > > call "copy_to_user()" and friends while holding the read lock.
> > > >
> > > > The bug appears to be in the implementation of the write semaphore -
> > > > down_write() doesn't undestand that blocked writes must not block
> > new
> > > > readers, exactly because of this situation.
> > >
> > > Exactly, same reason for which we need the same property from the rw
> > > spinlocks (to be allowed to read_lock without clearing irqs). Thanks
> > so
> > > much for reminding me about this! Unfortunately my rwsemaphores are
> > > blocking readers at the first down_write (for the better fairness
> > > property issuse, but I obviously forgotten that doing so I would
> > > introduce such a deadlock).
> >
> > i386 has a fair rwsemaphore, too - probably other archs must be modified
> > as well.
> 
> yes, actually my patch was against the rwsem patch in -aa, and in -aa
> I'm using the generic semaphores for all archs in the tree so it fixes
> the race for all them. The mainline semaphores are slightly different.

> if that's the very only place that could be a viable option but OTOH I
> like to be allowed to use recursion on the read locks as with the
> spinlocks. I think another option would be to have reacursion allowed on
> the default read locks and then make a down_read_fair that will block at
> if there's a down_write under us. we can very cleanly implement this,
> the same can be done cleanly also for the spinlocks: read_lock_fair. One
> can even mix the read_lock/read_lock_fair or the
> down_read/down_read_fair together. For example assuming we use the
> recursive semaphore fix in proc_pid_read_maps the down_read over there
> could be converted to a down_read_fair (but that's just an exercise, if
> the page fault isn't fair it doesn't worth to have proc_pid_read_maps
> fair either).

Be careful; If another user can grab your semaphore for read for a short 
time (eg for "top" or similar usage), he can construct several threads
that
do this in a busy loop; the end result is that this evil user is capable
of blocking out writers FOREVER if semaphores are unfair; nice DoS....