Re: [PATCH v2] fs/epoll: make nesting accounting safe for -rt kernel

[Davidlohr Bueso]

On Wed, 26 Feb 2020, Jason Baron wrote:

> Davidlohr Bueso pointed out that when CONFIG_DEBUG_LOCK_ALLOC is set
> ep_poll_safewake() can take several non-raw spinlocks after disabling
> interrupts. Since a spinlock can block in the -rt kernel, we can't take a
> spinlock after disabling interrupts. So let's re-work how we determine
> the nesting level such that it plays nicely with the -rt kernel.
>
> Let's introduce a 'nests' field in struct eventpoll that records the
> current nesting level during ep_poll_callback(). Then, if we nest again we
> can find the previous struct eventpoll that we were called from and
> increase our count by 1. The 'nests' field is protected by
> ep->poll_wait.lock.
>
> I've also moved the visited field to reduce the size of struct eventpoll
> from 184 bytes to 176 bytes on x86_64 for !CONFIG_DEBUG_LOCK_ALLOC,
> which is typical for a production config.
>
> Reported-by: Davidlohr Bueso <dbueso@xxxxxxx>
> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>

Sorry for the tardy reply. This looks good to me:

Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>