linux-kernel - RE: [PATCH] x86/mce/dev-mcelog: Fix potential memory access error

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Date:   Mon, 29 Mar 2021 16:12:16 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Yang Li <yang.lee@...ux.alibaba.com>
CC:     "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86/mce/dev-mcelog: Fix potential memory access error

-		set_bit(MCE_OVERFLOW, (unsigned long *)&mcelog->flags);
+		mcelog->flags |= BIT(MCE_OVERFLOW);

set_bit() is an atomic operation because it might race with the code to
get and clear this bit:

                do {
                        flags = mcelog->flags;
                } while (cmpxchg(&mcelog->flags, flags, 0) != flags);

Originally this was needed because mcelog could be called from #MC
context.

That's all changed now and the machine check notifier chain routines are
called in a kernel thread. So some other locking (mutex?) could be used
to protect access to mcelog->flags.

-Tony