netdev - Re: seqlock lockdep false positives?

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Date:   Tue, 9 Mar 2021 08:54:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Erhard F." <erhard_f@...lbox.org>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: seqlock lockdep false positives?

On Mon, Mar 08, 2021 at 09:42:08PM +0100, Erhard F. wrote:

> I can confirm that your patch on top of 5.12-rc2 makes the lockdep
> splat disappear (Ahmeds' 1st patch not installed).

Excellent, I'll queue the below in locking/urgent then.


---
Subject: u64_stats,lockdep: Fix u64_stats_init() vs lockdep
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon, 8 Mar 2021 09:38:12 +0100

Jakub reported that:

    static struct net_device *rtl8139_init_board(struct pci_dev *pdev)
    {
	    ...
	    u64_stats_init(&tp->rx_stats.syncp);
	    u64_stats_init(&tp->tx_stats.syncp);
	    ...
    }

results in lockdep getting confused between the RX and TX stats lock.
This is because u64_stats_init() is an inline calling seqcount_init(),
which is a macro using a static variable to generate a lockdep class.

By wrapping that in an inline, we negate the effect of the macro and
fold the static key variable, hence the confusion.

Fix by also making u64_stats_init() a macro for the case where it
matters, leaving the other case an inline for argument validation
etc.

Reported-by: Jakub Kicinski <kuba@...nel.org>
Debugged-by: "Ahmed S. Darwish" <a.darwish@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: "Erhard F." <erhard_f@...lbox.org>
Link: https://lkml.kernel.org/r/YEXicy6+9MksdLZh@hirez.programming.kicks-ass.net
---
 include/linux/u64_stats_sync.h |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -115,12 +115,13 @@ static inline void u64_stats_inc(u64_sta
 }
 #endif
 
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+#define u64_stats_init(syncp)	seqcount_init(&(syncp)->seq)
+#else
 static inline void u64_stats_init(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
-	seqcount_init(&syncp->seq);
-#endif
 }
+#endif
 
 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 {