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@xxxxxxx>
NeilBrown
>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
> 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
>
>
>
