nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or_lock()

[PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or_lock()

Posted by Oleg Nesterov 2 years, 1 month ago

The usage of read_seqbegin_or_lock() in nfsd_copy_write_verifier()
is wrong. "seq" is always even and thus "or_lock" has no effect,
this code can never take ->writeverf_lock for writing.

I guess this is fine, nfsd_copy_write_verifier() just copies 8 bytes
and nfsd_reset_write_verifier() is supposed to be very rare operation
so we do not need the adaptive locking in this case.

Yet the code looks wrong and sub-optimal, it can use read_seqbegin()
without changing the behaviour.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/nfsd/nfssvc.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index c7af1095f6b5..094b765c5397 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -359,13 +359,12 @@ static bool nfsd_needs_lockd(struct nfsd_net *nn)
  */
 void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn)
 {
-	int seq = 0;
+	unsigned seq;
 
 	do {
-		read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
+		seq = read_seqbegin(&nn->writeverf_lock);
 		memcpy(verf, nn->writeverf, sizeof(nn->writeverf));
-	} while (need_seqretry(&nn->writeverf_lock, seq));
-	done_seqretry(&nn->writeverf_lock, seq);
+	} while (read_seqretry(&nn->writeverf_lock, seq));
 }
 
 static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)
-- 
2.25.1.362.g51ebf55

Re: [PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or_lock()

Posted by NeilBrown 2 years, 1 month ago

On Fri, 27 Oct 2023, Oleg Nesterov wrote:
> The usage of read_seqbegin_or_lock() in nfsd_copy_write_verifier()
> is wrong. "seq" is always even and thus "or_lock" has no effect,
> this code can never take ->writeverf_lock for writing.
> 
> I guess this is fine, nfsd_copy_write_verifier() just copies 8 bytes
> and nfsd_reset_write_verifier() is supposed to be very rare operation
> so we do not need the adaptive locking in this case.
> 
> Yet the code looks wrong and sub-optimal, it can use read_seqbegin()
> without changing the behaviour.

Wow! read_seqbegin_or_lock() has never locked since

Commit: 88a411c07b6f ("seqlock: livelock fix") in Linux v2.6.26 (2008).

That's rather embarrassing.

I agree we don't need the lock on the read-side for nfsd.

Reviewed-by: NeilBrown <neilb@suse.de>

NeilBrown


> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  fs/nfsd/nfssvc.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index c7af1095f6b5..094b765c5397 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -359,13 +359,12 @@ static bool nfsd_needs_lockd(struct nfsd_net *nn)
>   */
>  void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn)
>  {
> -	int seq = 0;
> +	unsigned seq;
>  
>  	do {
> -		read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
> +		seq = read_seqbegin(&nn->writeverf_lock);
>  		memcpy(verf, nn->writeverf, sizeof(nn->writeverf));
> -	} while (need_seqretry(&nn->writeverf_lock, seq));
> -	done_seqretry(&nn->writeverf_lock, seq);
> +	} while (read_seqretry(&nn->writeverf_lock, seq));
>  }
>  
>  static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)
> -- 
> 2.25.1.362.g51ebf55
> 
> 
>

Re: [PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or_lock()

Posted by Jeff Layton 2 years, 1 month ago

On Thu, 2023-10-26 at 16:50 +0200, Oleg Nesterov wrote:
> The usage of read_seqbegin_or_lock() in nfsd_copy_write_verifier()
> is wrong. "seq" is always even and thus "or_lock" has no effect,
> this code can never take ->writeverf_lock for writing.
> 
> I guess this is fine, nfsd_copy_write_verifier() just copies 8 bytes
> and nfsd_reset_write_verifier() is supposed to be very rare operation
> so we do not need the adaptive locking in this case.
> 
> Yet the code looks wrong and sub-optimal, it can use read_seqbegin()
> without changing the behaviour.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  fs/nfsd/nfssvc.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index c7af1095f6b5..094b765c5397 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -359,13 +359,12 @@ static bool nfsd_needs_lockd(struct nfsd_net *nn)
>   */
>  void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn)
>  {
> -	int seq = 0;
> +	unsigned seq;
>  
>  	do {
> -		read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
> +		seq = read_seqbegin(&nn->writeverf_lock);
>  		memcpy(verf, nn->writeverf, sizeof(nn->writeverf));
> -	} while (need_seqretry(&nn->writeverf_lock, seq));
> -	done_seqretry(&nn->writeverf_lock, seq);
> +	} while (read_seqretry(&nn->writeverf_lock, seq));
>  }
>  
>  static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)

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