linux-kernel - Re: [PATCH] net: optimize cmpxchg in ip_idents

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Message-ID: <20200120081858.GI14879@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Jan 2020 09:18:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, jinyuqi@...wei.com,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org, edumazet@...gle.com,
        guoyang2@...wei.com, Will Deacon <will@...nel.org>
Subject: Re: [PATCH] net: optimize cmpxchg in ip_idents_reserve

On Fri, Jan 17, 2020 at 10:48:19AM -0800, Eric Dumazet wrote:
> 
> 
> On 1/17/20 10:38 AM, Arvind Sankar wrote:
> > On Fri, Jan 17, 2020 at 10:16:45AM -0800, Eric Dumazet wrote:
> >> Wasńt it the case back in 2016 already for linux-4.8 ?
> >>
> >> What will prevent someone to send another report to netdev/lkml ?
> >>
> >>  -fno-strict-overflow support is not a prereq for CONFIG_UBSAN.
> >>
> >> Fact that we kept in lib/ubsan.c and lib/test_ubsan.c code for 
> >> test_ubsan_add_overflow() and test_ubsan_sub_overflow() is disturbing.
> >>
> > 
> > No, it was bumped in 2018 in commit cafa0010cd51 ("Raise the minimum
> > required gcc version to 4.6"). That raised it from 3.2 -> 4.6.
> > 
> 
> This seems good to me, for gcc at least.
> 
> Maybe it is time to enfore -fno-strict-overflow in KBUILD_CFLAGS 
> instead of making it conditional.

IIRC there was a bug in UBSAN vs -fwrapv/-fno-strict-overflow that was
only fixed in gcc-8 or 9 or so.

So while the -fwrapv/-fno-strict-overflow flag has been correctly
supported since like forever, UBSAN was buggy until quite recent when
used in conjustion with that flag.