Re: [PATCH 7/8] mm/slab: introduce deferred submission of rcu sheaves

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 4/16/26 11:10, Harry Yoo (Oracle) wrote:
> Instead of falling back when the rcu sheaf becomes full, implement
> deferred submission of rcu sheaves. If kfree_rcu_sheaf() is invoked
> by kfree_rcu_nolock() (!allow_spin) and IRQs are disabled, the CPU might
> be in the middle of call_rcu() and thus defer call_rcu() with irq_work.

Seems like we're doing two things here. Implementing a fallback is the
advertised one. But also narrowing the definition of "unknown context where
call_rcu() is unsafe" to mean irqs_disabled().

Where does that condition (!irqs_disabled() means we're safe) come from
exactly, does RCU guarantee it anywhere? And if that condition is indeed
reliable, shouldn't it have been used by patch 5 already?

> Submit all deferred RCU sheaves to call_rcu() before calling
> rcu_barrier() to ensure the promise of kvfree_rcu_barrier().
> 
> An alternative approach could be to implement this in the RCU subsystem,
> tracking if it's safe to call call_rcu() and allowing falling back to
> deferred call_rcu() at the cost of more expensive rcu_barrier() calls.
> 
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>