Re: [PATCH net v3] net: stmmac: protect updates of 64-bit statistics counters

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Feb 13, 2024 at 03:51:35PM +0100, Eric Dumazet wrote:
> On Tue, Feb 13, 2024 at 3:29 PM Jisheng Zhang <jszhang@kernel.org> wrote:
> >
> > On Sun, Feb 11, 2024 at 08:30:21PM -0800, Guenter Roeck wrote:
> > > Hi,
> > >
> > > On Sat, Feb 03, 2024 at 08:09:27PM +0100, Petr Tesarik wrote:
> > > > As explained by a comment in <linux/u64_stats_sync.h>, write side of struct
> > > > u64_stats_sync must ensure mutual exclusion, or one seqcount update could
> > > > be lost on 32-bit platforms, thus blocking readers forever. Such lockups
> > > > have been observed in real world after stmmac_xmit() on one CPU raced with
> > > > stmmac_napi_poll_tx() on another CPU.
> > > >
> > > > To fix the issue without introducing a new lock, split the statics into
> > > > three parts:
> > > >
> > > > 1. fields updated only under the tx queue lock,
> > > > 2. fields updated only during NAPI poll,
> > > > 3. fields updated only from interrupt context,
> > > >
> > > > Updates to fields in the first two groups are already serialized through
> > > > other locks. It is sufficient to split the existing struct u64_stats_sync
> > > > so that each group has its own.
> > > >
> > > > Note that tx_set_ic_bit is updated from both contexts. Split this counter
> > > > so that each context gets its own, and calculate their sum to get the total
> > > > value in stmmac_get_ethtool_stats().
> > > >
> > > > For the third group, multiple interrupts may be processed by different CPUs
> > > > at the same time, but interrupts on the same CPU will not nest. Move fields
> > > > from this group to a newly created per-cpu struct stmmac_pcpu_stats.
> > > >
> > > > Fixes: 133466c3bbe1 ("net: stmmac: use per-queue 64 bit statistics where necessary")
> > > > Link: https://lore.kernel.org/netdev/Za173PhviYg-1qIn@torres.zugschlus.de/t/
> > > > Cc: stable@vger.kernel.org
> > > > Signed-off-by: Petr Tesarik <petr@tesarici.cz>
> > >
> > > This patch results in a lockdep splat. Backtrace and bisect results attached.
> > >
> > > Guenter
> > >
> > > ---
> > > [   33.736728] ================================
> > > [   33.736805] WARNING: inconsistent lock state
> > > [   33.736953] 6.8.0-rc4 #1 Tainted: G                 N
> > > [   33.737080] --------------------------------
> > > [   33.737155] inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
> > > [   33.737309] kworker/0:2/39 [HC1[1]:SC0[2]:HE0:SE0] takes:
> > > [   33.737459] ef792074 (&syncp->seq#2){?...}-{0:0}, at: sun8i_dwmac_dma_interrupt+0x9c/0x28c
> > > [   33.738206] {HARDIRQ-ON-W} state was registered at:
> > > [   33.738318]   lock_acquire+0x11c/0x368
> > > [   33.738431]   __u64_stats_update_begin+0x104/0x1ac
> > > [   33.738525]   stmmac_xmit+0x4d0/0xc58
> >
> > interesting lockdep splat...
> > stmmac_xmit() operates on txq_stats->q_syncp, while the
> > sun8i_dwmac_dma_interrupt() operates on pcpu's priv->xstats.pcpu_stats
> > they are different syncp. so how does lockdep splat happen.
> 
> Right, I do not see anything obvious yet.

Wild guess: I think it maybe saying that due to

	inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.

the critical code may somehow be interrupted and, while handling the
interrupt, try to acquire the same lock again.

Guenter