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Message-ID: <umfukiohyxcxxw5g6ca5g7stq7oonnr3sbvjyjshnbqalzffeq@2nrwqsmwcrug>
Date: Wed, 2 Apr 2025 13:56:06 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: torvalds@...ux-foundation.org, bpf@...r.kernel.org,
daniel@...earbox.net, andrii@...nel.org, martin.lau@...nel.org,
akpm@...ux-foundation.org, peterz@...radead.org, vbabka@...e.cz, bigeasy@...utronix.de,
rostedt@...dmis.org, mhocko@...e.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] locking/local_lock, mm: Replace localtry_ helpers
with local_trylock_t type
On Tue, Apr 01, 2025 at 01:52:45PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.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@...e.cz>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>
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