Re: [PATCH 3/5] locking/rwsem: add down_read[_killable]_nosteal()

[Peter Zijlstra]

On Tue, Nov 03, 2026 at 08:27:44AM +0000, Lorenzo Stoakes (ARM) wrote:
> Since commit 1a728dff855a ("locking/rwsem: Enable reader optimistic lock
> stealing") a sleeping, queued writer whose handoff timeout hasn't expired
> can have its lock stolen by a read lock issued before a woken queued writer
> was able to acquire a lock.
> 
> To avoid write lock starvation, this is only permitted if no other read
> locks are held.
> 
> However, this behaviour defeats the purpose of a reader periodically
> dropping the lock to allow writers in, as doing so only works if the write
> lock is acquired first, another read lock is held, or a queued writer's
> handoff timeout has expired.
> 
> Provide a means of indicating this intent via down_read_nosteal() and
> down_read_killable_nosteal().
> 
> Implement it by exposing a steal parameter to rwsem_down_read_slowpath(),
> update the starvation logic to modify this parameter and modify the steal
> branch to check it first.
> 
> Also drop the duplicated RWSEM_WRITER_LOCKED check and remove the
> goto as the steal condition makes this redundant.
> 
> Exposing this function is safe as it only makes the lock more pessimistic,
> so does not impact lock contention.

Not generally a fan of this -- can't we tweak the steal thing to never
steal if there are pending writers?