Re: [PATCH v4 net-next 01/14] locking/local_lock: Add local nested BH locking infrastructure.

[Peter Zijlstra]

On Tue, Jun 04, 2024 at 05:24:08PM +0200, Sebastian Andrzej Siewior wrote:
> Add local_lock_nested_bh() locking. It is based on local_lock_t and the
> naming follows the preempt_disable_nested() example.
> 
> For !PREEMPT_RT + !LOCKDEP it is a per-CPU annotation for locking
> assumptions based on local_bh_disable(). The macro is optimized away
> during compilation.
> For !PREEMPT_RT + LOCKDEP the local_lock_nested_bh() is reduced to
> the usual lock-acquire plus lockdep_assert_in_softirq() - ensuring that
> BH is disabled.
> 
> For PREEMPT_RT local_lock_nested_bh() acquires the specified per-CPU
> lock. It does not disable CPU migration because it relies on
> local_bh_disable() disabling CPU migration.

should we assert this? lockdep_assert(current->migration_disabled) or
somesuch should do, rite?

> With LOCKDEP it performans the usual lockdep checks as with !PREEMPT_RT.
> Due to include hell the softirq check has been moved spinlock.c.
> 
> The intention is to use this locking in places where locking of a per-CPU
> variable relies on BH being disabled. Instead of treating disabled
> bottom halves as a big per-CPU lock, PREEMPT_RT can use this to reduce
> the locking scope to what actually needs protecting.
> A side effect is that it also documents the protection scope of the
> per-CPU variables.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

Otherwise I suppose sp.. not a fan of the whole nested thing, but I
don't really have an alternative proposal so yeah, whatever :-)