[PATCHv2] nvme: use srcu for iterating namespace list

[Shinichiro Kawasaki]

On May 23, 2024 / 10:20, Keith Busch wrote:
> From: Keith Busch <kbusch at kernel.org>
> 
> The nvme pci driver synchronizes with all the namespace queues during a
> reset to ensure that there's no pending timeout work.
> 
> Meanwhile the timeout work potentially iterates those same namespaces to
> freeze their queues.
> 
> Each of those namespace iterations use the same read lock. If a write
> lock should somehow get between the synchronize and freeze steps, then
> forward progress is deadlocked.
> 
> We had been relying on the nvme controller state machine to ensure the
> reset work wouldn't conflict with timeout work. That guarantee may be a
> bit fragile to rely on, so iterate the namespace lists without taking a
> lock to fix potential circular locks, as reported by lockdep.
> 
> Link: https://lore.kernel.org/all/20220930001943.zdbvolc3gkekfmcv@shindev/
> Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki at wdc.com>
> Signed-off-by: Keith Busch <kbusch at kernel.org>

Keith, thank you very much for the patch. Sagi, Christoph, thank you for the
discussion.

I confirmed this patch avoids the lockdep WARN that I reported. I also ran my
test set with the patch and observed no regression. Looks good.

Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki at wdc.com>