Re: [PATCH v2] locking/local_lock, mm: Replace localtry_ helpers with local_trylock_t type

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Apr 01, 2025 at 01:52:45PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Partially revert commit 0aaddfb06882 ("locking/local_lock: Introduce localtry_lock_t").
> Remove localtry_*() helpers, since localtry_lock() name might
> be misinterpreted as "try lock".
> 
> Introduce local_trylock[_irqsave]() helpers that only work
> with newly introduced local_trylock_t type.
> Note that attempt to use local_trylock[_irqsave]() with local_lock_t
> will cause compilation failure.
> 
> Usage and behavior in !PREEMPT_RT:
> 
> local_lock_t lock;                     // sizeof(lock) == 0
> local_lock(&lock);                     // preempt disable
> local_lock_irqsave(&lock, ...);        // irq save
> if (local_trylock_irqsave(&lock, ...)) // compilation error
> 
> local_trylock_t lock;                  // sizeof(lock) == 4

Is there a reason for this 'acquired' to be int? Can it be uint8_t? No
need to change anything here but I plan to change it later to compact as
much as possible within one (or two) cachline for memcg stocks.

> local_lock(&lock);                     // preempt disable, acquired = 1
> local_lock_irqsave(&lock, ...);        // irq save, acquired = 1
> if (local_trylock(&lock))              // if (!acquired) preempt disable
> if (local_trylock_irqsave(&lock, ...)) // if (!acquired) irq save

For above two ", acquired = 1" as well.

> 
> The existing local_lock_*() macros can be used either with
> local_lock_t or local_trylock_t.
> With local_trylock_t they set acquired = 1 while local_unlock_*() clears it.
> 
> In !PREEMPT_RT local_lock_irqsave(local_lock_t *) disables interrupts
> to protect critical section, but it doesn't prevent NMI, so the fully
> reentrant code cannot use local_lock_irqsave(local_lock_t *) for
> exclusive access.
> 
> The local_lock_irqsave(local_trylock_t *) helper disables interrupts
> and sets acquired=1, so local_trylock_irqsave(local_trylock_t *) from
> NMI attempting to acquire the same lock will return false.
> 
> In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().
> Map local_trylock_irqsave() to preemptible spin_trylock().
> When in hard IRQ or NMI return false right away, since
> spin_trylock() is not safe due to explicit locking in the underneath
> rt_spin_trylock() implementation. Removing this explicit locking and
> attempting only "trylock" is undesired due to PI implications.
> 
> The local_trylock() without _irqsave can be used to avoid the cost of
> disabling/enabling interrupts by only disabling preemption, so
> local_trylock() in an interrupt attempting to acquire the same
> lock will return false.
> 
> Note there is no need to use local_inc for acquired variable,
> since it's a percpu variable with strict nesting scopes.
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>