linux-kernel - Re: [PATCH v4] btrfs: add fs state details to error messages.

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Message-ID: <Yhfnx21q51xSBl32@relinquished.localdomain>
Date:   Thu, 24 Feb 2022 12:17:11 -0800
From:   Omar Sandoval <osandov@...ndov.com>
To:     Sweet Tea Dorminy <sweettea-kernel@...miny.me>
Cc:     dsterba@...e.cz, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v4] btrfs: add fs state details to error messages.

On Thu, Feb 24, 2022 at 03:09:08PM -0500, Sweet Tea Dorminy wrote:
> > > All the other interactions with info->fs_state are test/set/clear_bit,
> > > which treat the argument as volatile and are therefore safe to do from
> > > multiple threads. Without the READ_ONCE (reading it as a volatile),
> > > the compiler or cpu could turn the reads of info->fs_state in
> > > for_each_set_bit() into writes of random stuff into info->fs_state,
> > > potentially clearing the state bits or filling them with garbage.
> > I'm not sure I'm missing something, but I find the above hard to
> > believe. Concurrent access to a variable from multiple threads may not
> > produce consistent results, but random writes should not happen when
> > we're just reading.
> 
> Maybe I've been reading too many articles about the things compilers are
> technically allowed to do. But as per the following link, the C standard
> does permit compilers inventing writes except to atomics and volatiles:
> https://lwn.net/Articles/793253/#Invented%20Stores
> 
> > 
> > > Even if this is right, it'd be rare, but it would be exceeding weird
> > > for a message to be logged listing an error and then future messages
> > > be logged without any such state, or with a random collection of
> > > garbage states.
> > How would that happen? The volatile keyword is only a compiler hint not
> > to do optimizations on the variable, what actually happens on the CPU
> > level depends if the instruction is locked or not, so different threads
> > may read different bits.
> > You seem to imply that once a variable is not used with volatile
> > semantics, even just for read, the result could lead to random writes
> > because it's otherwise undefined.
> 
> Pretty much; once a variable is read without READ_ONCE, it's unsafe to write
> a new value on another thread that depends on the old value. Imagine a
> compiler which invents stores; then if you are both reading and setting a
> variable 'a' on different threads, the following could happen:
> 
> thread 1 (reads)       thread 2 (modifies)
> 
> reads a into tmp
> 
> stores junk into a
> 
>                                 reads junk from a
> 
> stores tmp into a
> 
>                                 writes junk | 2 to a
> 
> 
> Now a contains junk indefinitely.
> 
> 
> But if it's too theoretical, I'm happy to drop it and amend my paranoia
> level.

I agree with Sweet Tea here. Even if it's very theoretical, it costs us
nothing to do the "correct" thing here.