linux-kernel - Re: Build regressions/improvements in v5.18-rc1

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Date:   Tue, 5 Apr 2022 08:16:05 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: Build regressions/improvements in v5.18-rc1

On Mon, Apr 04, 2022 at 01:45:05PM +0200, Arnd Bergmann wrote:
> On Mon, Apr 4, 2022 at 12:19 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> 
> > >
> > > /kisskb/src/fs/xfs/./xfs_trace.h:432:2: note: in expansion of macro 'TP_printk'
> > >   TP_printk("dev %d:%d daddr 0x%llx bbcount 0x%x hold %d pincount %d "
> > >   ^
> > > /kisskb/src/fs/xfs/./xfs_trace.h:440:5: note: in expansion of macro '__print_flags'
> > >      __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
> > >      ^
> > > /kisskb/src/fs/xfs/xfs_buf.h:67:4: note: in expansion of macro 'XBF_UNMAPPED'
> > >   { XBF_UNMAPPED,  "UNMAPPED" }
> > >     ^
> > > /kisskb/src/fs/xfs/./xfs_trace.h:440:40: note: in expansion of macro 'XFS_BUF_FLAGS'
> > >      __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
> > >                                         ^
> > > /kisskb/src/fs/xfs/./xfs_trace.h: In function 'trace_raw_output_xfs_buf_flags_class':
> > > /kisskb/src/fs/xfs/xfs_buf.h:46:23: error: initializer element is not constant
> > >  #define XBF_UNMAPPED  (1 << 31)/* do not map the buffer */
> > >
> > > This doesn't make a whole lotta sense to me. It's blown up in a
> > > tracepoint macro in XFS that was not changed at all in 5.18-rc1, nor
> > > was any of the surrounding XFS code or contexts.  Perhaps something
> > > outside XFS changed to cause this on these platforms?
> >
> > Upon closer look, all builds showing this issue are using gcc-5...
> >
> > > Can you bisect this, please?
> >
> > Fortunately I still have gcc-5 installed on an older machine,
> > and I could reproduce the issue on amd64 with
> > "make allmodconfig fs/xfs/xfs_trace.o".
> >
> > Bisection points to commit e8c07082a810fbb9 ("Kbuild: move to
> > -std=gnu11").
> >
> > [1] gcc version 5.5.0 20171010 (Ubuntu 5.5.0-12ubuntu1
> 
> Thanks for the report. I've produced it and can see that the problem
> is assigning
> the value of "(1 << 31)" to an 'unsigned long' struct member. Since this is
> a signed integer overflow, the result is technically undefined behavior,
> which gcc-5 does not accept as an integer constant.
> 
> The patch below fixes it for me, but I have not checked if there are any
> other instances. This could also be done using the 'BIT()' macro if the
> XFS maintainers prefer:

So XFS only uses these flags in unsigned int fields that are
typed via:

typedef unsigned int xfs_buf_flags_t;

So on the surface, declaring the flag values as ULONG and then writing
them into a UINT field is not a nice thing to be doing.

I really don't want to change the xfs_buf_flags_t type to an
unsigned long, because that changes the packing of the first
cacheline of the struct xfs_buf and the contents of that cacheline
are performance critical for the lookup fastpath....

Looking at __print_flags, the internal array type declaration is:

struct trace_print_flags {
        unsigned long           mask;
        const char              *name;
};

and that's the source of the problem.  I notice __print_flags_u64()
exists, but __print_flags_u32() does not. Should it?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com