Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light"atomic readers to prevent CPU offline

[Steven Rostedt]

On Mon, 2012-12-10 at 19:21 +0100, Oleg Nesterov wrote:
> On 12/07, Oleg Nesterov wrote:
> >
> > On 12/06, Steven Rostedt wrote:
> > >
> > > You know reader locks can deadlock with each other, right? And this
> > > isn't caught be lockdep yet. This is because rwlocks have been made to
> > > be fair with writers. Before writers could be starved if a CPU always
> > > let a reader in. Now if a writer is waiting, a reader will block behind
> > > the writer. But this has introduced new issues with the kernel as
> > > follows:
> > >
> > >
> > >    CPU0			   CPU1	 	   CPU2		   CPU3
> > >    ----			   ----		   ----		   ----
> > > read_lock(A);
> > > 			read_lock(B)
> > > 					write_lock(A) <- block
> > > 							write_lock(B) <- block
> > > read_lock(B) <-block
> > >
> > > 			read_lock(A) <- block
> > >
> > > DEADLOCK!
> >
> > Really??? Oh I didn't know...
> >
> > Yes this was always true for rwsem, but rwlock_t?
> 
> Sorry, please ignore my email. I misread your email.
> 

No prob, looking at what I wrote, I should have explicitly stated two
different rwlocks. The only hint that I gave about two locks was (A) and
(B). Even what I started with: "reader locks can deadlock with each
other" is a bit ambiguous. So I can easily see the confusion.

-- Steve


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