Re: xfstests 073 regression

[Linus Torvalds]

On Sun, Jul 31, 2011 at 1:47 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> Yes, I already have, a couple of hours before you sent this:
>
> http://www.spinics.net/lists/linux-fsdevel/msg47357.html
>
> We haven't found the root cause of the problem, and writeback cannot
> hold off grab_super_passive() because writeback only holds read
> locks on s_umount, just like grab_super_passive.

With read-write semaphores, even read-vs-read recursion is a deadlock
possibility.

Why? Because if a writer comes in on another thread, while the read
lock is initially held, then the writer will now block. And due to
fairness, now a subsequent reader will *also* block.

So no, nesting readers is *not* allowed for rw_semaphores even if
naively you'd think it should work.

So if xfstests 073 does mount/umount testing, then it is entirely
possible that a reader blocks another reader due to a pending writer.

NOTE! The rwlock *spinlocks* are designed to be unfair to writers, and
by design allow recursive readers. That's important and very much by
design: it is ok to take a rwlock for reading without disabling
interrupts even if there may be *interrupts* that also need it for
reading.

With the spinning rwlock, there is also much less chance of starvation
due to this unfairness. In contrast, the rw_semaphores really can be
starved pretty easily if you are excessively unfair to writers.

                          Linus
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