Re: Updated patch for the the lock_sem deadlock

[David Wysochanski <dwysocha@redhat.com>]

On Fri, Oct 25, 2019 at 12:24 PM Aurélien Aptel <aaptel@suse.com> wrote:
>
> Ronnie Sahlberg <lsahlber@redhat.com> writes:
> > This is a small update to Dave's patch to address Pavels recommendation
> > that we use a helper function for the trylock/sleep loop.
>
> Disclamer: I have not read all the emails regarding this patch but it
> is not obvious to me how replacing
>
>     lock()
>
> by
>
>     while (trylock())
>         sleep()
>
> is fixing things, but I'm sure I'm missing something :(
>

Let me try to explain better.

The deadlock occurs because of how rw_semaphores work in Linux.

The deadlock occurred because we had:
1. thread1: down_read() and obtained the semaphore
2. thread2: down_write() blocked due to thread1
3. thread1: down_read (a second time), blocked due to thread2

Note that it is normally benign for a single thread to call down_read
twice.  However, in this case, another thread called down_write in
between the two calls.  Once one thread calls down_write, any callers
of down_read will block, that is the rw_semaphore implementation in
Linux.  If this was not the case, we could have callers of down_read
continually streaming in and starving out callers of down_write.

The patch removes thread2 from blocking, so #3 will never occur, hence
removing the deadlock.