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Date:   Fri, 4 Feb 2022 20:02:26 -0800
From:   Sultan Alsawaf <sultan@kerneltoast.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Theodore Ts'o <tytso@mit.edu>,
        Jonathan =?iso-8859-1?Q?Neusch=E4fer?= <j.neuschaefer@gmx.net>
Subject: Re: [PATCH RFC v1] random: do not take spinlocks in irq handler
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On Fri, Feb 04, 2022 at 09:47:23PM +0100, Sebastian Andrzej Siewior wrote:
> No need for atomic. If this is truly per-CPU then there will be no
> cross-CPU access, right?
> Therefore I would suggest to use __this_cpu_inc_return() which would avoid
> the sync prefix for the inc operation. Same for __this_cpu_or(). And you
> could use unsigned int.

Hi,

The __this_cpu_{ATOMIC_OP}() functions are for atomically performing a single
per-CPU operation for the current CPU from contexts that permit CPU migration.
Since this code is safe from CPU migrations (add_interrupt_randomness() runs in
hardirq context), the atomic per-CPU helpers are unneeded. Instead of using
__this_cpu_inc_return() and __this_cpu_or(), we can operate on the per-CPU
pointer directly without any extra safety (e.g., `++fast_pool->count` can be
used in place of `__this_cpu_inc_return(irq_randomness.count)`).

Sultan