Re: [PATCH] fs/dcache: use lockdep assertion instead of warn try_lock

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2022-03-25 at 20:00 +0100, Niels Dossche wrote:
> Currently, there is a fallback with a WARN that uses down_read_trylock
> as a safety measure for when there is no lock taken. The current
> callsites expect a write lock to be taken. Moreover, the s_root field
> is written to, which is not allowed under a read lock.
> This code safety fallback should not be executed unless there is an
> issue somewhere else.
> Using a lockdep assertion better communicates the intent of the code,
> and gets rid of the currently slightly wrong fallback solution.
> 
> Note:
> I am currently working on a static analyser to detect missing locks
> using type-based static analysis as my master's thesis
> in order to obtain my master's degree.
> If you would like to have more details, please let me know.
> This was a reported case. I manually verified the report by looking
> at the code, so that I do not send wrong information or patches.
> After concluding that this seems to be a true positive, I created
> this patch. I have both compile-tested this patch and runtime-tested
> this patch on x86_64. The effect on a running system could be a
> potential race condition in exceptional cases.
> This issue was found on Linux v5.17.
> 
> Fixes: c636ebdb186bf ("VFS: Destroy the dentries contributed by a superblock on unmounting")
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
> ---
>  fs/dcache.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index c84269c6e8bf..0142f15340e5 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1692,7 +1692,7 @@ void shrink_dcache_for_umount(struct super_block *sb)
>  {
>  	struct dentry *dentry;
>  
> -	WARN(down_read_trylock(&sb->s_umount), "s_umount should've been locked");
> +	lockdep_assert_held_write(&sb->s_umount);
>  
>  	dentry = sb->s_root;
>  	sb->s_root = NULL;

Much nicer.

Reviewed-by: Jeff Layton <jlayton@kernel.org>