From: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
To: "Igor Malovitsa" <igor.malovitsa@gmail.com>, <bpf@vger.kernel.org>
Cc: <daniel@iogearbox.net>, <andrii@kernel.org>,
<martin.lau@linux.dev>, <eddyz87@gmail.com>, <song@kernel.org>,
<yonghong.song@linux.dev>, <jolsa@kernel.org>, <memxor@gmail.com>,
<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf 1/2] bpf: Load ringbuf consumer_pos under the spinlock
Date: Fri, 16 Oct 2026 16:14:44 +0000 [thread overview]
Message-ID: <DM6EA4NZEW6T.1QXD6YWB6J9JZ@gmail.com> (raw)
In-Reply-To: <20261016003318.529860-2-igor.malovitsa@gmail.com>
On Fri, Oct 16, 2026 at 12:33 AM Igor Malovitsa <igor.malovitsa@gmail.com> wrote:
> - cons_pos = smp_load_acquire(&rb->consumer_pos);
> -
> if (raw_res_spin_lock_irqsave(&rb->spinlock, flags))
> return NULL;
>
> + /* Under the lock, so at most one ring behind producer_pos. */
> + cons_pos = smp_load_acquire(&rb->consumer_pos);
> +
libbpf writes consumer_pos after every record, so this load is
a cache miss most of the time.
Today it's done before the lock. With this patch all other producers
spin on the lock while this cpu waits for it. On 64-bit too.
So "Neither changes anything on 64-bit" in the cover letter is not correct.
imo it's not worth slowing down everyone for that.
pw-bot: cr
next prev parent reply other threads:[~2026-10-16 16:14 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-16 0:33 [PATCH bpf 0/2] bpf: Fix ring buffer checks on 32-bit position wrap Igor Malovitsa
2026-10-16 0:33 ` [PATCH bpf 1/2] bpf: Load ringbuf consumer_pos under the spinlock Igor Malovitsa
2026-10-16 16:14 ` Alexei Starovoitov [this message]
2026-10-16 19:12 ` Igor Malovitsa
2026-10-16 0:33 ` [PATCH bpf 2/2] bpf: Fix user ringbuf empty check on 32-bit position wrap Igor Malovitsa
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