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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 19 Jun 2022 09:50:32 -0500
Message-ID: <CAHk-=wgfrUdWBXA-Jx7ZC1x3wwsomou0L6niGgpRS2Hd5rQxyg@mail.gmail.com>
Subject: Re: [GIT pull] locking/urgent for 5.19-rc3
To:     Thomas Gleixner <tglx@linutronix.de>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Sun, Jun 19, 2022 at 8:12 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> A RT fix for lockdep. lockdep invokes prandom_u32() to create cookies. This
> worked until prandom_u32() was switched to the real random generator, which
> takes a spinlock for extraction, which does not work on RT when invoked
> from atomic contexts. lockdep has no requirement for real random numbers
> and it turns out sched_clock() is good enough to create the cookie. That
> works everywhere and is faster.

So this is obviously fine and works ok, but I do think it highlights
that maybe that prandom change was a bad bad idea.

Even outside of RT, you might end up getting nasty locks within locks.
Not a deadlock, but a "this was just pointless".

                 Linus