linux-kernel - Re: [RFC 2/5] fs: freeze on suspend and thaw on resume

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Message-ID: <1507062758.2567.22.camel@wdc.com>
Date:   Tue, 3 Oct 2017 20:32:39 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "mcgrof@...nel.org" <mcgrof@...nel.org>
CC:     "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "ONeukum@...e.com" <ONeukum@...e.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
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        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
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Subject: Re: [RFC 2/5] fs: freeze on suspend and thaw on resume

On Tue, 2017-10-03 at 22:23 +0200, Luis R. Rodriguez wrote:
> On Tue, Oct 03, 2017 at 08:02:22PM +0000, Bart Van Assche wrote:
> > On Tue, 2017-10-03 at 11:53 -0700, Luis R. Rodriguez wrote:
> > > +static bool super_allows_freeze(struct super_block *sb)
> > > +{
> > > +	return !!(sb->s_type->fs_flags & FS_FREEZE_ON_SUSPEND);
> > > +}
> > 
> > A minor comment: if "!!" would be left out the compiler will perform the
> > conversion from int to bool implicitly
> 
> For all compilers?

Let's have a look at the output of the following commands:

$ PAGER= git grep 'typedef.*[[:blank:]]bool;' include
include/linux/types.h:typedef _Bool                     bool;
$ PAGER= git grep std= Makefile
Makefile:               -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS)
Makefile:                  -std=gnu89 $(call cc-option,-fno-PIE)

From https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/C-Dialect-Options.html#C-Dialect-Options:
‘gnu89’
GNU dialect of ISO C90 (including some C99 features).

I think this means that the Linux kernel tree can only be compiled correctly
by compilers that support the C11 type _Bool.

> > Anyway, I agree with the approach of this patch and I think
> > that freezing filesystems before processes are frozen would be a big step
> > forward.
> 
> Great! But please note, the current implementation calls fs_suspend_freeze()
> *after* try_to_freeze_tasks(), ie: this implementation freezes userspace and
> only after then filesystems.

What will the impact be of that choice on filesystems implemented in userspace?

Thanks,

Bart.