Re: [PATCH v2] nfsd: Always lock state exclusively.

J . Bruce Fields
2016-06-14T18:46:57Z

On Tue, Jun 14, 2016 at 11:56:20AM -0400, Oleg Drokin wrote:
>
> On Jun 14, 2016, at 11:46 AM, J . Bruce Fields wrote:
>
> > On Sun, Jun 12, 2016 at 09:26:27PM -0400, Oleg Drokin wrote:
> >> It used to be the case that state had an rwlock that was locked for write
> >> by downgrades, but for read for upgrades (opens). Well, the problem is
> >> if there are two competing opens for the same state, they step on
> >> each other toes potentially leading to leaking file descriptors
> >> from the state structure, since access mode is a bitmap only set once.
> >>
> >> Extend the holding region around in nfsd4_process_open2() to avoid
> >> racing entry into nfs4_get_vfs_file().
> >> Make init_open_stateid() return with locked stateid to be unlocked
> >> by the caller.
> >>
> >> Now this version held up pretty well in my testing for 24 hours.
> >> It still does not address the situation if during one of the racing
> >> nfs4_get_vfs_file() calls we are getting an error from one (first?)
> >> of them. This is to be addressed in a separate patch after having a
> >> solid reproducer (potentially using some fault injection).
> >>
> >> Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
> >> ---
> >> fs/nfsd/nfs4state.c | 47 +++++++++++++++++++++++++++--------------------
> >> fs/nfsd/state.h | 2 +-
> >> 2 files changed, 28 insertions(+), 21 deletions(-)
> >>
> >> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> >> index f5f82e1..fa5fb5a 100644
> >> --- a/fs/nfsd/nfs4state.c
> >> +++ b/fs/nfsd/nfs4state.c
> >> @@ -3487,6 +3487,10 @@ init_open_stateid(struct nfs4_ol_stateid *stp, struct nfs4_file *fp,
> >> struct nfs4_openowner *oo = open->op_openowner;
> >> struct nfs4_ol_stateid *retstp = NULL;
> >>
> >> + /* We are moving these outside of the spinlocks to avoid the warnings */
> >> + mutex_init(&stp->st_mutex);
> >> + mutex_lock(&stp->st_mutex);
> >
> > A mutex_init_locked() primitive might also be convenient here.
>
> I know! I would be able to do it under spinlock then without moving this around too.
>
> But alas, not only there is not one, mutex documentation states this is disallowed.

You're just talking about this comment?:

* It is not allowed to initialize an already locked mutex.

That's a weird comment. You're proably right that what they meant was
something like "It is not allowed to initialize a mutex to locked
state". But, I don't know, taken literally that comment doesn't make
sense (how could you even distinguish between an already-locked mutex
and an uninitialized mutex?), so maybe it'd be worth asking.

> > You could also take the two previous lines from the caller into this
> > function instead of passing in stp, that might simplify the code.
> > (Haven't checked.)
>
> I am not really sure what do you mean here.
> These lines are moved from further away in this function )well, just the init, anyway).
>
> Having half initialisation of stp here and half in the caller sounds kind of strange
> to me.

I was thinking of something like the following--so init_open_stateid
hides more of the details of the swapping. Untested. Does it look like
an improvement to you?

There's got to be a way to make this code a little less convoluted....

--b.

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index fa5fb5aa4847..41b59854c40f 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -3480,13 +3480,15 @@ alloc_init_open_stateowner(unsigned int strhashval, struct nfsd4_open *open,
}

static struct nfs4_ol_stateid *
-init_open_stateid(struct nfs4_ol_stateid *stp, struct nfs4_file *fp,
- struct nfsd4_open *open)
+init_open_stateid(struct nfs4_file *fp, struct nfsd4_open *open)
{

struct nfs4_openowner *oo = open->op_openowner;
struct nfs4_ol_stateid *retstp = NULL;
+ struct nfs4_ol_stateid *stp;

+ stp = open->op_stp;
+ open->op_stp = NULL;
/* We are moving these outside of the spinlocks to avoid the warnings */
mutex_init(&stp->st_mutex);
mutex_lock(&stp->st_mutex);
@@ -3512,9 +3514,12 @@ init_open_stateid(struct nfs4_ol_stateid *stp, struct nfs4_file *fp,
out_unlock:
spin_unlock(&fp->fi_lock);
spin_unlock(&oo->oo_owner.so_client->cl_lock);
- if (retstp)
- mutex_lock(&retstp->st_mutex);
- return retstp;
+ if (retstp) {
+ nfs4_put_stid(&stp->st_stid);
+ stp = retstp;
+ mutex_lock(&stp->st_mutex);
+ }
+ return stp;
}

/*
@@ -4310,7 +4315,6 @@ nfsd4_process_open2(struct svc_rqst *rqstp, struct svc_fh *current_fh, struct nf
struct nfs4_client *cl = open->op_openowner->oo_owner.so_client;
struct nfs4_file *fp = NULL;
struct nfs4_ol_stateid *stp = NULL;
- struct nfs4_ol_stateid *swapstp = NULL;
struct nfs4_delegation *dp = NULL;
__be32 status;

@@ -4347,16 +4351,9 @@ nfsd4_process_open2(struct svc_rqst *rqstp, struct svc_fh *current_fh, struct nf
goto out;
}
} else {
- stp = open->op_stp;
- open->op_stp = NULL;
- /*
- * init_open_stateid() either returns a locked stateid
- * it found, or initializes and locks the new one we passed in
- */
- swapstp = init_open_stateid(stp, fp, open);
- if (swapstp) {
- nfs4_put_stid(&stp->st_stid);
- stp = swapstp;
+ /* stp is returned locked: */
+ stp = init_open_stateid(fp, open);
+ if (stp->st_access_bmap == 0) {
status = nfs4_upgrade_open(rqstp, fp, current_fh,
stp, open);
if (status) {