[PATCH 0/2 v2] nfsd: fix handling of error from unshare_fs_struct()

[PATCH 0/2 v2] nfsd: fix handling of error from unshare_fs_struct()

[NeilBrown]

This v2 applies correctly to  git/cel/linux#master.

These two patches replace my previous patch:
 [PATCH 07/14] Change unshare_fs_struct() to never fail.

I had explored ways to change kthread_create() to avoid the need for
GFP_NOFAIL and concluded that we can do everything we need in the sunrpc
layer.  So the first patch here is a simple cleanup, and the second adds
simple infrastructure for an svc thread to confirm that it has started
up and to report if it was successful in that.

Thanks,
NeilBrown

 [PATCH 1/2] sunrpc: merge svc_rqst_alloc() into svc_prepare_thread()
 [PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[PATCH 1/2] sunrpc: merge svc_rqst_alloc() into svc_prepare_thread()

[NeilBrown]

The only caller of svc_rqst_alloc() is svc_prepare_thread().  So merge
the one into the other and simplify.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 net/sunrpc/svc.c | 25 +++++++------------------
 1 file changed, 7 insertions(+), 18 deletions(-)

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index e3e7c514a30c..ae31ffea7a14 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -678,7 +678,7 @@ svc_rqst_free(struct svc_rqst *rqstp)
 }
 
 static struct svc_rqst *
-svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
+svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 {
 	struct svc_rqst	*rqstp;
 
@@ -706,21 +706,6 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
 	if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
 		goto out_enomem;
 
-	return rqstp;
-out_enomem:
-	svc_rqst_free(rqstp);
-	return NULL;
-}
-
-static struct svc_rqst *
-svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
-{
-	struct svc_rqst	*rqstp;
-
-	rqstp = svc_rqst_alloc(serv, pool, node);
-	if (!rqstp)
-		return ERR_PTR(-ENOMEM);
-
 	serv->sv_nrthreads += 1;
 	pool->sp_nrthreads += 1;
 
@@ -730,6 +715,10 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 	list_add_rcu(&rqstp->rq_all, &pool->sp_all_threads);
 
 	return rqstp;
+
+out_enomem:
+	svc_rqst_free(rqstp);
+	return NULL;
 }
 
 /**
@@ -810,8 +799,8 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 		node = svc_pool_map_get_node(chosen_pool->sp_id);
 
 		rqstp = svc_prepare_thread(serv, chosen_pool, node);
-		if (IS_ERR(rqstp))
-			return PTR_ERR(rqstp);
+		if (!rqstp)
+			return -ENOMEM;
 		task = kthread_create_on_node(serv->sv_threadfn, rqstp,
 					      node, "%s", serv->sv_name);
 		if (IS_ERR(task)) {
-- 
2.44.0

[PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[NeilBrown]

If an svc thread needs to perform some initialisation that might fail,
it has no good way to handle the failure.

Before the thread can exit it must call svc_exit_thread(), but that
requires the service mutex to be held.  The thread cannot simply take
the mutex as that could deadlock if there is a concurrent attempt to
shut down all threads (which is unlikely, but not impossible).

nfsd currently call svc_exit_thread() unprotected in the unlikely event
that unshare_fs_struct() fails.

We can clean this up by introducing svc_thread_init_status() by which an
svc thread can report whether initialisation has succeeded.  If it has,
it continues normally into the action loop.  If it has not,
svc_thread_init_status() immediately aborts the thread.
svc_start_kthread() waits for either of these to happen, and calls
svc_exit_thread() (under the mutex) if the thread aborted.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/lockd/svc.c             |  2 ++
 fs/nfs/callback.c          |  2 ++
 fs/nfsd/nfssvc.c           |  9 +++------
 include/linux/sunrpc/svc.h | 28 ++++++++++++++++++++++++++++
 net/sunrpc/svc.c           | 11 +++++++++++
 5 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index 71713309967d..4ec22c2f2ea3 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -124,6 +124,8 @@ lockd(void *vrqstp)
 	struct net *net = &init_net;
 	struct lockd_net *ln = net_generic(net, lockd_net_id);
 
+	svc_thread_init_status(rqstp, 0);
+
 	/* try_to_freeze() is called from svc_recv() */
 	set_freezable();
 
diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
index 8adfcd4c8c1a..6cf92498a5ac 100644
--- a/fs/nfs/callback.c
+++ b/fs/nfs/callback.c
@@ -76,6 +76,8 @@ nfs4_callback_svc(void *vrqstp)
 {
 	struct svc_rqst *rqstp = vrqstp;
 
+	svc_thread_init_status(rqstp, 0);
+
 	set_freezable();
 
 	while (!svc_thread_should_stop(rqstp))
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index fca340b3ee91..1cef09a3c78e 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -875,11 +875,9 @@ nfsd(void *vrqstp)
 
 	/* At this point, the thread shares current->fs
 	 * with the init process. We need to create files with the
-	 * umask as defined by the client instead of init's umask. */
-	if (unshare_fs_struct() < 0) {
-		printk("Unable to start nfsd thread: out of memory\n");
-		goto out;
-	}
+	 * umask as defined by the client instead of init's umask.
+	 */
+	svc_thread_init_status(rqstp, unshare_fs_struct());
 
 	current->fs->umask = 0;
 
@@ -901,7 +899,6 @@ nfsd(void *vrqstp)
 
 	atomic_dec(&nfsd_th_cnt);
 
-out:
 	/* Release the thread */
 	svc_exit_thread(rqstp);
 	return 0;
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 99e9345d829e..437672bcaa22 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -21,6 +21,7 @@
 #include <linux/wait.h>
 #include <linux/mm.h>
 #include <linux/pagevec.h>
+#include <linux/kthread.h>
 
 /*
  *
@@ -232,6 +233,11 @@ struct svc_rqst {
 	struct net		*rq_bc_net;	/* pointer to backchannel's
 						 * net namespace
 						 */
+
+	int			rq_err;		/* Thread sets this to inidicate
+						 * initialisation success.
+						 */
+
 	unsigned long	bc_to_initval;
 	unsigned int	bc_to_retries;
 	void **			rq_lease_breaker; /* The v4 client breaking a lease */
@@ -305,6 +311,28 @@ static inline bool svc_thread_should_stop(struct svc_rqst *rqstp)
 	return test_bit(RQ_VICTIM, &rqstp->rq_flags);
 }
 
+/**
+ * svc_thread_init_status - report whether thread has initialised successfully
+ * @rqstp: the thread in question
+ * @err: errno code
+ *
+ * After performing any initialisation that could fail, and before starting
+ * normal work, each sunrpc svc_thread must call svc_thread_init_status()
+ * with an appropriate error, or zero.
+ *
+ * If zero is passed, the thread is ready and must continue until
+ * svc_thread_should_stop() returns true.  If a non-zero error is passed
+ * the call will not return - the thread will exit.
+ */
+static inline void svc_thread_init_status(struct svc_rqst *rqstp, int err)
+{
+	/* store_release ensures svc_start_kthreads() sees the error */
+	smp_store_release(&rqstp->rq_err, err);
+	wake_up_var(&rqstp->rq_err);
+	if (err)
+		kthread_exit(1);
+}
+
 struct svc_deferred_req {
 	u32			prot;	/* protocol (UDP or TCP) */
 	struct svc_xprt		*xprt;
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index ae31ffea7a14..1e80fa67d8b7 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -706,6 +706,8 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 	if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
 		goto out_enomem;
 
+	rqstp->rq_err = -EAGAIN; /* No error yet */
+
 	serv->sv_nrthreads += 1;
 	pool->sp_nrthreads += 1;
 
@@ -792,6 +794,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 	struct svc_pool *chosen_pool;
 	unsigned int state = serv->sv_nrthreads-1;
 	int node;
+	int err;
 
 	do {
 		nrservs--;
@@ -814,6 +817,14 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 
 		svc_sock_update_bufs(serv);
 		wake_up_process(task);
+
+		/* load_acquire ensures we get value stored in svc_thread_init_status() */
+		wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
+		err = rqstp->rq_err;
+		if (err) {
+			svc_exit_thread(rqstp);
+			return err;
+		}
 	} while (nrservs > 0);
 
 	return 0;
-- 
2.44.0

Re: [PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[Jeff Layton]

On Tue, 2024-07-30 at 07:19 +1000, NeilBrown wrote:
> If an svc thread needs to perform some initialisation that might fail,
> it has no good way to handle the failure.
> 
> Before the thread can exit it must call svc_exit_thread(), but that
> requires the service mutex to be held.  The thread cannot simply take
> the mutex as that could deadlock if there is a concurrent attempt to
> shut down all threads (which is unlikely, but not impossible).
> 
> nfsd currently call svc_exit_thread() unprotected in the unlikely event
> that unshare_fs_struct() fails.
> 
> We can clean this up by introducing svc_thread_init_status() by which an
> svc thread can report whether initialisation has succeeded.  If it has,
> it continues normally into the action loop.  If it has not,
> svc_thread_init_status() immediately aborts the thread.
> svc_start_kthread() waits for either of these to happen, and calls
> svc_exit_thread() (under the mutex) if the thread aborted.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/lockd/svc.c             |  2 ++
>  fs/nfs/callback.c          |  2 ++
>  fs/nfsd/nfssvc.c           |  9 +++------
>  include/linux/sunrpc/svc.h | 28 ++++++++++++++++++++++++++++
>  net/sunrpc/svc.c           | 11 +++++++++++
>  5 files changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
> index 71713309967d..4ec22c2f2ea3 100644
> --- a/fs/lockd/svc.c
> +++ b/fs/lockd/svc.c
> @@ -124,6 +124,8 @@ lockd(void *vrqstp)
>  	struct net *net = &init_net;
>  	struct lockd_net *ln = net_generic(net, lockd_net_id);
>  
> +	svc_thread_init_status(rqstp, 0);
> +
>  	/* try_to_freeze() is called from svc_recv() */
>  	set_freezable();
>  
> diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
> index 8adfcd4c8c1a..6cf92498a5ac 100644
> --- a/fs/nfs/callback.c
> +++ b/fs/nfs/callback.c
> @@ -76,6 +76,8 @@ nfs4_callback_svc(void *vrqstp)
>  {
>  	struct svc_rqst *rqstp = vrqstp;
>  
> +	svc_thread_init_status(rqstp, 0);
> +
>  	set_freezable();
>  
>  	while (!svc_thread_should_stop(rqstp))
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index fca340b3ee91..1cef09a3c78e 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -875,11 +875,9 @@ nfsd(void *vrqstp)
>  
>  	/* At this point, the thread shares current->fs
>  	 * with the init process. We need to create files with the
> -	 * umask as defined by the client instead of init's umask. */
> -	if (unshare_fs_struct() < 0) {
> -		printk("Unable to start nfsd thread: out of memory\n");
> -		goto out;
> -	}
> +	 * umask as defined by the client instead of init's umask.
> +	 */
> +	svc_thread_init_status(rqstp, unshare_fs_struct());
>  
>  	current->fs->umask = 0;
>  
> @@ -901,7 +899,6 @@ nfsd(void *vrqstp)
>  
>  	atomic_dec(&nfsd_th_cnt);
>  
> -out:
>  	/* Release the thread */
>  	svc_exit_thread(rqstp);
>  	return 0;
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 99e9345d829e..437672bcaa22 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -21,6 +21,7 @@
>  #include <linux/wait.h>
>  #include <linux/mm.h>
>  #include <linux/pagevec.h>
> +#include <linux/kthread.h>
>  
>  /*
>   *
> @@ -232,6 +233,11 @@ struct svc_rqst {
>  	struct net		*rq_bc_net;	/* pointer to backchannel's
>  						 * net namespace
>  						 */
> +
> +	int			rq_err;		/* Thread sets this to inidicate
> +						 * initialisation success.
> +						 */
> +
>  	unsigned long	bc_to_initval;
>  	unsigned int	bc_to_retries;
>  	void **			rq_lease_breaker; /* The v4 client breaking a lease */
> @@ -305,6 +311,28 @@ static inline bool svc_thread_should_stop(struct svc_rqst *rqstp)
>  	return test_bit(RQ_VICTIM, &rqstp->rq_flags);
>  }
>  
> +/**
> + * svc_thread_init_status - report whether thread has initialised successfully
> + * @rqstp: the thread in question
> + * @err: errno code
> + *
> + * After performing any initialisation that could fail, and before starting
> + * normal work, each sunrpc svc_thread must call svc_thread_init_status()
> + * with an appropriate error, or zero.
> + *
> + * If zero is passed, the thread is ready and must continue until
> + * svc_thread_should_stop() returns true.  If a non-zero error is passed
> + * the call will not return - the thread will exit.
> + */
> +static inline void svc_thread_init_status(struct svc_rqst *rqstp, int err)
> +{
> +	/* store_release ensures svc_start_kthreads() sees the error */
> +	smp_store_release(&rqstp->rq_err, err);
> +	wake_up_var(&rqstp->rq_err);
> +	if (err)
> +		kthread_exit(1);
> +}
> +
>  struct svc_deferred_req {
>  	u32			prot;	/* protocol (UDP or TCP) */
>  	struct svc_xprt		*xprt;
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index ae31ffea7a14..1e80fa67d8b7 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -706,6 +706,8 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
>  	if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
>  		goto out_enomem;
>  
> +	rqstp->rq_err = -EAGAIN; /* No error yet */
> +
>  	serv->sv_nrthreads += 1;
>  	pool->sp_nrthreads += 1;
>  
> @@ -792,6 +794,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
>  	struct svc_pool *chosen_pool;
>  	unsigned int state = serv->sv_nrthreads-1;
>  	int node;
> +	int err;
>  
>  	do {
>  		nrservs--;
> @@ -814,6 +817,14 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
>  
>  		svc_sock_update_bufs(serv);
>  		wake_up_process(task);
> +
> +		/* load_acquire ensures we get value stored in svc_thread_init_status() */
> +		wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
> +		err = rqstp->rq_err;
> +		if (err) {
> +			svc_exit_thread(rqstp);
> +			return err;
> +		}
>  	} while (nrservs > 0);
>  
>  	return 0;


I hit a hang today on the client while running the nfs-interop test
under kdevops. The client is stuck in mount syscall, while trying to
set up the backchannel:

[ 1693.653257] INFO: task mount.nfs:13243 blocked for more than 120 seconds.
[ 1693.655827]       Not tainted 6.11.0-rc7-gffcadb41b696 #166
[ 1693.657858] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1693.661442] task:mount.nfs       state:D stack:0     pid:13243 tgid:13243 ppid:13242  flags:0x00004002
[ 1693.664648] Call Trace:
[ 1693.665639]  <TASK>
[ 1693.666482]  __schedule+0xc04/0x2750
[ 1693.668021]  ? __pfx___schedule+0x10/0x10
[ 1693.669562]  ? _raw_spin_lock_irqsave+0x98/0xf0
[ 1693.671213]  ? __pfx__raw_spin_lock_irqsave+0x10/0x10
[ 1693.673109]  ? try_to_wake_up+0x141/0x1210
[ 1693.674763]  ? __pfx_try_to_wake_up+0x10/0x10
[ 1693.676612]  ? _raw_spin_unlock_irqrestore+0x23/0x40
[ 1693.678429]  schedule+0x73/0x2f0
[ 1693.679662]  svc_set_num_threads+0xbc8/0xf80 [sunrpc]
[ 1693.682007]  ? __pfx_svc_set_num_threads+0x10/0x10 [sunrpc]
[ 1693.684163]  ? __pfx_var_wake_function+0x10/0x10
[ 1693.685849]  ? __svc_create+0x6c0/0x980 [sunrpc]
[ 1693.687777]  nfs_callback_up+0x314/0xae0 [nfsv4]
[ 1693.689630]  nfs4_init_client+0x203/0x400 [nfsv4]
[ 1693.691468]  ? __pfx_nfs4_init_client+0x10/0x10 [nfsv4]
[ 1693.693502]  ? _raw_spin_lock_irqsave+0x98/0xf0
[ 1693.695141]  nfs4_set_client+0x2f4/0x520 [nfsv4]
[ 1693.696967]  ? __pfx_nfs4_set_client+0x10/0x10 [nfsv4]
[ 1693.699230]  nfs4_create_server+0x5f2/0xef0 [nfsv4]
[ 1693.701357]  ? _raw_spin_lock+0x85/0xe0
[ 1693.702758]  ? __pfx__raw_spin_lock+0x10/0x10
[ 1693.704344]  ? nfs_get_tree+0x61f/0x16a0 [nfs]
[ 1693.706160]  ? __pfx_nfs4_create_server+0x10/0x10 [nfsv4]
[ 1693.707376]  ? __module_get+0x26/0xc0
[ 1693.708061]  nfs4_try_get_tree+0xcd/0x250 [nfsv4]
[ 1693.708893]  vfs_get_tree+0x83/0x2d0
[ 1693.709534]  path_mount+0x88d/0x19a0
[ 1693.710100]  ? __pfx_path_mount+0x10/0x10
[ 1693.710718]  ? user_path_at+0xa4/0xe0
[ 1693.711303]  ? kmem_cache_free+0x143/0x3e0
[ 1693.711936]  __x64_sys_mount+0x1fb/0x270
[ 1693.712606]  ? __pfx___x64_sys_mount+0x10/0x10
[ 1693.713315]  do_syscall_64+0x4b/0x110
[ 1693.713889]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 1693.714660] RIP: 0033:0x7f05050418fe
[ 1693.715233] RSP: 002b:00007fff4e0f5728 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
[ 1693.716417] RAX: ffffffffffffffda RBX: 00007fff4e0f5900 RCX: 00007f05050418fe
[ 1693.717492] RDX: 00005559cea3ea70 RSI: 00005559cea3fe60 RDI: 00005559cea40140
[ 1693.718558] RBP: 00007fff4e0f5760 R08: 00005559cea41e60 R09: 00007f050510cb20
[ 1693.719620] R10: 0000000000000000 R11: 0000000000000246 R12: 00005559cea41e60
[ 1693.720735] R13: 00007fff4e0f5900 R14: 00005559cea41df0 R15: 0000000000000004


Looking at faddr2line:

$ ./scripts/faddr2line --list net/sunrpc/sunrpc.ko svc_set_num_threads+0xbc8/0xf80
svc_set_num_threads+0xbc8/0xf80:

svc_start_kthreads at /home/jlayton/git/kdevops/linux/net/sunrpc/svc.c:822 (discriminator 17)
 817 
 818                    svc_sock_update_bufs(serv);
 819                    wake_up_process(task);
 820 
 821                    /* load_acquire ensures we get value stored in svc_thread_init_status() */
>822<                   wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
 823                    err = rqstp->rq_err;
 824                    if (err) {
 825                            svc_exit_thread(rqstp);
 826                            return err;
 827                    }

(inlined by) svc_set_num_threads at /home/jlayton/git/kdevops/linux/net/sunrpc/svc.c:877 (discriminator 17)
 872                    nrservs -= serv->sv_nrthreads;
 873            else
 874                    nrservs -= pool->sp_nrthreads;
 875 
 876            if (nrservs > 0)
>877<                   return svc_start_kthreads(serv, pool, nrservs);
 878            if (nrservs < 0)
 879                    return svc_stop_kthreads(serv, pool, nrservs);
 880            return 0;
 881    }
 882    EXPORT_SYMBOL_GPL(svc_set_num_threads);

It looks like the callback thread started up properly and is past the svc_thread_init_status call.

$ sudo cat /proc/13246/stack
[<0>] svc_recv+0xcef/0x2020 [sunrpc]
[<0>] nfs4_callback_svc+0xb0/0x140 [nfsv4]
[<0>] kthread+0x29b/0x360
[<0>] ret_from_fork+0x30/0x70
[<0>] ret_from_fork_asm+0x1a/0x30

...so the status should have been updated properly. Barrier problem?
-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[Chuck Lever III]

> On Sep 11, 2024, at 2:33 PM, Jeff Layton <jlayton@kernel.org> wrote:
> 
> On Tue, 2024-07-30 at 07:19 +1000, NeilBrown wrote:
>> If an svc thread needs to perform some initialisation that might fail,
>> it has no good way to handle the failure.
>> 
>> Before the thread can exit it must call svc_exit_thread(), but that
>> requires the service mutex to be held.  The thread cannot simply take
>> the mutex as that could deadlock if there is a concurrent attempt to
>> shut down all threads (which is unlikely, but not impossible).
>> 
>> nfsd currently call svc_exit_thread() unprotected in the unlikely event
>> that unshare_fs_struct() fails.
>> 
>> We can clean this up by introducing svc_thread_init_status() by which an
>> svc thread can report whether initialisation has succeeded.  If it has,
>> it continues normally into the action loop.  If it has not,
>> svc_thread_init_status() immediately aborts the thread.
>> svc_start_kthread() waits for either of these to happen, and calls
>> svc_exit_thread() (under the mutex) if the thread aborted.
>> 
>> Signed-off-by: NeilBrown <neilb@suse.de>
>> ---
>> fs/lockd/svc.c             |  2 ++
>> fs/nfs/callback.c          |  2 ++
>> fs/nfsd/nfssvc.c           |  9 +++------
>> include/linux/sunrpc/svc.h | 28 ++++++++++++++++++++++++++++
>> net/sunrpc/svc.c           | 11 +++++++++++
>> 5 files changed, 46 insertions(+), 6 deletions(-)
>> 
>> diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
>> index 71713309967d..4ec22c2f2ea3 100644
>> --- a/fs/lockd/svc.c
>> +++ b/fs/lockd/svc.c
>> @@ -124,6 +124,8 @@ lockd(void *vrqstp)
>> struct net *net = &init_net;
>> struct lockd_net *ln = net_generic(net, lockd_net_id);
>> 
>> + svc_thread_init_status(rqstp, 0);
>> +
>> /* try_to_freeze() is called from svc_recv() */
>> set_freezable();
>> 
>> diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
>> index 8adfcd4c8c1a..6cf92498a5ac 100644
>> --- a/fs/nfs/callback.c
>> +++ b/fs/nfs/callback.c
>> @@ -76,6 +76,8 @@ nfs4_callback_svc(void *vrqstp)
>> {
>> struct svc_rqst *rqstp = vrqstp;
>> 
>> + svc_thread_init_status(rqstp, 0);
>> +
>> set_freezable();
>> 
>> while (!svc_thread_should_stop(rqstp))
>> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
>> index fca340b3ee91..1cef09a3c78e 100644
>> --- a/fs/nfsd/nfssvc.c
>> +++ b/fs/nfsd/nfssvc.c
>> @@ -875,11 +875,9 @@ nfsd(void *vrqstp)
>> 
>> /* At this point, the thread shares current->fs
>>  * with the init process. We need to create files with the
>> -  * umask as defined by the client instead of init's umask. */
>> - if (unshare_fs_struct() < 0) {
>> - printk("Unable to start nfsd thread: out of memory\n");
>> - goto out;
>> - }
>> +  * umask as defined by the client instead of init's umask.
>> +  */
>> + svc_thread_init_status(rqstp, unshare_fs_struct());
>> 
>> current->fs->umask = 0;
>> 
>> @@ -901,7 +899,6 @@ nfsd(void *vrqstp)
>> 
>> atomic_dec(&nfsd_th_cnt);
>> 
>> -out:
>> /* Release the thread */
>> svc_exit_thread(rqstp);
>> return 0;
>> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
>> index 99e9345d829e..437672bcaa22 100644
>> --- a/include/linux/sunrpc/svc.h
>> +++ b/include/linux/sunrpc/svc.h
>> @@ -21,6 +21,7 @@
>> #include <linux/wait.h>
>> #include <linux/mm.h>
>> #include <linux/pagevec.h>
>> +#include <linux/kthread.h>
>> 
>> /*
>>  *
>> @@ -232,6 +233,11 @@ struct svc_rqst {
>> struct net *rq_bc_net; /* pointer to backchannel's
>>  * net namespace
>>  */
>> +
>> + int rq_err; /* Thread sets this to inidicate
>> +  * initialisation success.
>> +  */
>> +
>> unsigned long bc_to_initval;
>> unsigned int bc_to_retries;
>> void ** rq_lease_breaker; /* The v4 client breaking a lease */
>> @@ -305,6 +311,28 @@ static inline bool svc_thread_should_stop(struct svc_rqst *rqstp)
>> return test_bit(RQ_VICTIM, &rqstp->rq_flags);
>> }
>> 
>> +/**
>> + * svc_thread_init_status - report whether thread has initialised successfully
>> + * @rqstp: the thread in question
>> + * @err: errno code
>> + *
>> + * After performing any initialisation that could fail, and before starting
>> + * normal work, each sunrpc svc_thread must call svc_thread_init_status()
>> + * with an appropriate error, or zero.
>> + *
>> + * If zero is passed, the thread is ready and must continue until
>> + * svc_thread_should_stop() returns true.  If a non-zero error is passed
>> + * the call will not return - the thread will exit.
>> + */
>> +static inline void svc_thread_init_status(struct svc_rqst *rqstp, int err)
>> +{
>> + /* store_release ensures svc_start_kthreads() sees the error */
>> + smp_store_release(&rqstp->rq_err, err);
>> + wake_up_var(&rqstp->rq_err);
>> + if (err)
>> + kthread_exit(1);
>> +}
>> +
>> struct svc_deferred_req {
>> u32 prot; /* protocol (UDP or TCP) */
>> struct svc_xprt *xprt;
>> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
>> index ae31ffea7a14..1e80fa67d8b7 100644
>> --- a/net/sunrpc/svc.c
>> +++ b/net/sunrpc/svc.c
>> @@ -706,6 +706,8 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
>> if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
>> goto out_enomem;
>> 
>> + rqstp->rq_err = -EAGAIN; /* No error yet */
>> +
>> serv->sv_nrthreads += 1;
>> pool->sp_nrthreads += 1;
>> 
>> @@ -792,6 +794,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
>> struct svc_pool *chosen_pool;
>> unsigned int state = serv->sv_nrthreads-1;
>> int node;
>> + int err;
>> 
>> do {
>> nrservs--;
>> @@ -814,6 +817,14 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
>> 
>> svc_sock_update_bufs(serv);
>> wake_up_process(task);
>> +
>> + /* load_acquire ensures we get value stored in svc_thread_init_status() */
>> + wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
>> + err = rqstp->rq_err;
>> + if (err) {
>> + svc_exit_thread(rqstp);
>> + return err;
>> + }
>> } while (nrservs > 0);
>> 
>> return 0;
> 
> 
> I hit a hang today on the client while running the nfs-interop test
> under kdevops. The client is stuck in mount syscall, while trying to
> set up the backchannel:
> 
> [ 1693.653257] INFO: task mount.nfs:13243 blocked for more than 120 seconds.
> [ 1693.655827]       Not tainted 6.11.0-rc7-gffcadb41b696 #166
> [ 1693.657858] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1693.661442] task:mount.nfs       state:D stack:0     pid:13243 tgid:13243 ppid:13242  flags:0x00004002
> [ 1693.664648] Call Trace:
> [ 1693.665639]  <TASK>
> [ 1693.666482]  __schedule+0xc04/0x2750
> [ 1693.668021]  ? __pfx___schedule+0x10/0x10
> [ 1693.669562]  ? _raw_spin_lock_irqsave+0x98/0xf0
> [ 1693.671213]  ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> [ 1693.673109]  ? try_to_wake_up+0x141/0x1210
> [ 1693.674763]  ? __pfx_try_to_wake_up+0x10/0x10
> [ 1693.676612]  ? _raw_spin_unlock_irqrestore+0x23/0x40
> [ 1693.678429]  schedule+0x73/0x2f0
> [ 1693.679662]  svc_set_num_threads+0xbc8/0xf80 [sunrpc]
> [ 1693.682007]  ? __pfx_svc_set_num_threads+0x10/0x10 [sunrpc]
> [ 1693.684163]  ? __pfx_var_wake_function+0x10/0x10
> [ 1693.685849]  ? __svc_create+0x6c0/0x980 [sunrpc]
> [ 1693.687777]  nfs_callback_up+0x314/0xae0 [nfsv4]
> [ 1693.689630]  nfs4_init_client+0x203/0x400 [nfsv4]
> [ 1693.691468]  ? __pfx_nfs4_init_client+0x10/0x10 [nfsv4]
> [ 1693.693502]  ? _raw_spin_lock_irqsave+0x98/0xf0
> [ 1693.695141]  nfs4_set_client+0x2f4/0x520 [nfsv4]
> [ 1693.696967]  ? __pfx_nfs4_set_client+0x10/0x10 [nfsv4]
> [ 1693.699230]  nfs4_create_server+0x5f2/0xef0 [nfsv4]
> [ 1693.701357]  ? _raw_spin_lock+0x85/0xe0
> [ 1693.702758]  ? __pfx__raw_spin_lock+0x10/0x10
> [ 1693.704344]  ? nfs_get_tree+0x61f/0x16a0 [nfs]
> [ 1693.706160]  ? __pfx_nfs4_create_server+0x10/0x10 [nfsv4]
> [ 1693.707376]  ? __module_get+0x26/0xc0
> [ 1693.708061]  nfs4_try_get_tree+0xcd/0x250 [nfsv4]
> [ 1693.708893]  vfs_get_tree+0x83/0x2d0
> [ 1693.709534]  path_mount+0x88d/0x19a0
> [ 1693.710100]  ? __pfx_path_mount+0x10/0x10
> [ 1693.710718]  ? user_path_at+0xa4/0xe0
> [ 1693.711303]  ? kmem_cache_free+0x143/0x3e0
> [ 1693.711936]  __x64_sys_mount+0x1fb/0x270
> [ 1693.712606]  ? __pfx___x64_sys_mount+0x10/0x10
> [ 1693.713315]  do_syscall_64+0x4b/0x110
> [ 1693.713889]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> [ 1693.714660] RIP: 0033:0x7f05050418fe
> [ 1693.715233] RSP: 002b:00007fff4e0f5728 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> [ 1693.716417] RAX: ffffffffffffffda RBX: 00007fff4e0f5900 RCX: 00007f05050418fe
> [ 1693.717492] RDX: 00005559cea3ea70 RSI: 00005559cea3fe60 RDI: 00005559cea40140
> [ 1693.718558] RBP: 00007fff4e0f5760 R08: 00005559cea41e60 R09: 00007f050510cb20
> [ 1693.719620] R10: 0000000000000000 R11: 0000000000000246 R12: 00005559cea41e60
> [ 1693.720735] R13: 00007fff4e0f5900 R14: 00005559cea41df0 R15: 0000000000000004
> 
> 
> Looking at faddr2line:
> 
> $ ./scripts/faddr2line --list net/sunrpc/sunrpc.ko svc_set_num_threads+0xbc8/0xf80
> svc_set_num_threads+0xbc8/0xf80:
> 
> svc_start_kthreads at /home/jlayton/git/kdevops/linux/net/sunrpc/svc.c:822 (discriminator 17)
> 817 
> 818                    svc_sock_update_bufs(serv);
> 819                    wake_up_process(task);
> 820 
> 821                    /* load_acquire ensures we get value stored in svc_thread_init_status() */
>> 822<                   wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
> 823                    err = rqstp->rq_err;
> 824                    if (err) {
> 825                            svc_exit_thread(rqstp);
> 826                            return err;
> 827                    }
> 
> (inlined by) svc_set_num_threads at /home/jlayton/git/kdevops/linux/net/sunrpc/svc.c:877 (discriminator 17)
> 872                    nrservs -= serv->sv_nrthreads;
> 873            else
> 874                    nrservs -= pool->sp_nrthreads;
> 875 
> 876            if (nrservs > 0)
>> 877<                   return svc_start_kthreads(serv, pool, nrservs);
> 878            if (nrservs < 0)
> 879                    return svc_stop_kthreads(serv, pool, nrservs);
> 880            return 0;
> 881    }
> 882    EXPORT_SYMBOL_GPL(svc_set_num_threads);
> 
> It looks like the callback thread started up properly and is past the svc_thread_init_status call.
> 
> $ sudo cat /proc/13246/stack
> [<0>] svc_recv+0xcef/0x2020 [sunrpc]
> [<0>] nfs4_callback_svc+0xb0/0x140 [nfsv4]
> [<0>] kthread+0x29b/0x360
> [<0>] ret_from_fork+0x30/0x70
> [<0>] ret_from_fork_asm+0x1a/0x30
> 
> ...so the status should have been updated properly. Barrier problem?

Speaking as NFSD release manager: I don't intend to send the
NFSD v6.12 PR until week of September 23rd.

I will either drop these patches before sending my PR, or I
can squash in a fix if one is found.


--
Chuck Lever

Re: [PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[NeilBrown]

On Sat, 14 Sep 2024, Chuck Lever III wrote:
> 
> 
> > On Sep 11, 2024, at 2:33 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Tue, 2024-07-30 at 07:19 +1000, NeilBrown wrote:
> >> If an svc thread needs to perform some initialisation that might fail,
> >> it has no good way to handle the failure.
> >> 
> >> Before the thread can exit it must call svc_exit_thread(), but that
> >> requires the service mutex to be held.  The thread cannot simply take
> >> the mutex as that could deadlock if there is a concurrent attempt to
> >> shut down all threads (which is unlikely, but not impossible).
> >> 
> >> nfsd currently call svc_exit_thread() unprotected in the unlikely event
> >> that unshare_fs_struct() fails.
> >> 
> >> We can clean this up by introducing svc_thread_init_status() by which an
> >> svc thread can report whether initialisation has succeeded.  If it has,
> >> it continues normally into the action loop.  If it has not,
> >> svc_thread_init_status() immediately aborts the thread.
> >> svc_start_kthread() waits for either of these to happen, and calls
> >> svc_exit_thread() (under the mutex) if the thread aborted.
> >> 
> >> Signed-off-by: NeilBrown <neilb@suse.de>
> >> ---
> >> fs/lockd/svc.c             |  2 ++
> >> fs/nfs/callback.c          |  2 ++
> >> fs/nfsd/nfssvc.c           |  9 +++------
> >> include/linux/sunrpc/svc.h | 28 ++++++++++++++++++++++++++++
> >> net/sunrpc/svc.c           | 11 +++++++++++
> >> 5 files changed, 46 insertions(+), 6 deletions(-)
> >> 
> >> diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
> >> index 71713309967d..4ec22c2f2ea3 100644
> >> --- a/fs/lockd/svc.c
> >> +++ b/fs/lockd/svc.c
> >> @@ -124,6 +124,8 @@ lockd(void *vrqstp)
> >> struct net *net = &init_net;
> >> struct lockd_net *ln = net_generic(net, lockd_net_id);
> >> 
> >> + svc_thread_init_status(rqstp, 0);
> >> +
> >> /* try_to_freeze() is called from svc_recv() */
> >> set_freezable();
> >> 
> >> diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
> >> index 8adfcd4c8c1a..6cf92498a5ac 100644
> >> --- a/fs/nfs/callback.c
> >> +++ b/fs/nfs/callback.c
> >> @@ -76,6 +76,8 @@ nfs4_callback_svc(void *vrqstp)
> >> {
> >> struct svc_rqst *rqstp = vrqstp;
> >> 
> >> + svc_thread_init_status(rqstp, 0);
> >> +
> >> set_freezable();
> >> 
> >> while (!svc_thread_should_stop(rqstp))
> >> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> >> index fca340b3ee91..1cef09a3c78e 100644
> >> --- a/fs/nfsd/nfssvc.c
> >> +++ b/fs/nfsd/nfssvc.c
> >> @@ -875,11 +875,9 @@ nfsd(void *vrqstp)
> >> 
> >> /* At this point, the thread shares current->fs
> >>  * with the init process. We need to create files with the
> >> -  * umask as defined by the client instead of init's umask. */
> >> - if (unshare_fs_struct() < 0) {
> >> - printk("Unable to start nfsd thread: out of memory\n");
> >> - goto out;
> >> - }
> >> +  * umask as defined by the client instead of init's umask.
> >> +  */
> >> + svc_thread_init_status(rqstp, unshare_fs_struct());
> >> 
> >> current->fs->umask = 0;
> >> 
> >> @@ -901,7 +899,6 @@ nfsd(void *vrqstp)
> >> 
> >> atomic_dec(&nfsd_th_cnt);
> >> 
> >> -out:
> >> /* Release the thread */
> >> svc_exit_thread(rqstp);
> >> return 0;
> >> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> >> index 99e9345d829e..437672bcaa22 100644
> >> --- a/include/linux/sunrpc/svc.h
> >> +++ b/include/linux/sunrpc/svc.h
> >> @@ -21,6 +21,7 @@
> >> #include <linux/wait.h>
> >> #include <linux/mm.h>
> >> #include <linux/pagevec.h>
> >> +#include <linux/kthread.h>
> >> 
> >> /*
> >>  *
> >> @@ -232,6 +233,11 @@ struct svc_rqst {
> >> struct net *rq_bc_net; /* pointer to backchannel's
> >>  * net namespace
> >>  */
> >> +
> >> + int rq_err; /* Thread sets this to inidicate
> >> +  * initialisation success.
> >> +  */
> >> +
> >> unsigned long bc_to_initval;
> >> unsigned int bc_to_retries;
> >> void ** rq_lease_breaker; /* The v4 client breaking a lease */
> >> @@ -305,6 +311,28 @@ static inline bool svc_thread_should_stop(struct svc_rqst *rqstp)
> >> return test_bit(RQ_VICTIM, &rqstp->rq_flags);
> >> }
> >> 
> >> +/**
> >> + * svc_thread_init_status - report whether thread has initialised successfully
> >> + * @rqstp: the thread in question
> >> + * @err: errno code
> >> + *
> >> + * After performing any initialisation that could fail, and before starting
> >> + * normal work, each sunrpc svc_thread must call svc_thread_init_status()
> >> + * with an appropriate error, or zero.
> >> + *
> >> + * If zero is passed, the thread is ready and must continue until
> >> + * svc_thread_should_stop() returns true.  If a non-zero error is passed
> >> + * the call will not return - the thread will exit.
> >> + */
> >> +static inline void svc_thread_init_status(struct svc_rqst *rqstp, int err)
> >> +{
> >> + /* store_release ensures svc_start_kthreads() sees the error */
> >> + smp_store_release(&rqstp->rq_err, err);
> >> + wake_up_var(&rqstp->rq_err);
> >> + if (err)
> >> + kthread_exit(1);
> >> +}
> >> +
> >> struct svc_deferred_req {
> >> u32 prot; /* protocol (UDP or TCP) */
> >> struct svc_xprt *xprt;
> >> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> >> index ae31ffea7a14..1e80fa67d8b7 100644
> >> --- a/net/sunrpc/svc.c
> >> +++ b/net/sunrpc/svc.c
> >> @@ -706,6 +706,8 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
> >> if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
> >> goto out_enomem;
> >> 
> >> + rqstp->rq_err = -EAGAIN; /* No error yet */
> >> +
> >> serv->sv_nrthreads += 1;
> >> pool->sp_nrthreads += 1;
> >> 
> >> @@ -792,6 +794,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
> >> struct svc_pool *chosen_pool;
> >> unsigned int state = serv->sv_nrthreads-1;
> >> int node;
> >> + int err;
> >> 
> >> do {
> >> nrservs--;
> >> @@ -814,6 +817,14 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
> >> 
> >> svc_sock_update_bufs(serv);
> >> wake_up_process(task);
> >> +
> >> + /* load_acquire ensures we get value stored in svc_thread_init_status() */
> >> + wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
> >> + err = rqstp->rq_err;
> >> + if (err) {
> >> + svc_exit_thread(rqstp);
> >> + return err;
> >> + }
> >> } while (nrservs > 0);
> >> 
> >> return 0;
> > 
> > 
> > I hit a hang today on the client while running the nfs-interop test
> > under kdevops. The client is stuck in mount syscall, while trying to
> > set up the backchannel:
> > 
> > [ 1693.653257] INFO: task mount.nfs:13243 blocked for more than 120 seconds.
> > [ 1693.655827]       Not tainted 6.11.0-rc7-gffcadb41b696 #166
> > [ 1693.657858] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [ 1693.661442] task:mount.nfs       state:D stack:0     pid:13243 tgid:13243 ppid:13242  flags:0x00004002
> > [ 1693.664648] Call Trace:
> > [ 1693.665639]  <TASK>
> > [ 1693.666482]  __schedule+0xc04/0x2750
> > [ 1693.668021]  ? __pfx___schedule+0x10/0x10
> > [ 1693.669562]  ? _raw_spin_lock_irqsave+0x98/0xf0
> > [ 1693.671213]  ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> > [ 1693.673109]  ? try_to_wake_up+0x141/0x1210
> > [ 1693.674763]  ? __pfx_try_to_wake_up+0x10/0x10
> > [ 1693.676612]  ? _raw_spin_unlock_irqrestore+0x23/0x40
> > [ 1693.678429]  schedule+0x73/0x2f0
> > [ 1693.679662]  svc_set_num_threads+0xbc8/0xf80 [sunrpc]
> > [ 1693.682007]  ? __pfx_svc_set_num_threads+0x10/0x10 [sunrpc]
> > [ 1693.684163]  ? __pfx_var_wake_function+0x10/0x10
> > [ 1693.685849]  ? __svc_create+0x6c0/0x980 [sunrpc]
> > [ 1693.687777]  nfs_callback_up+0x314/0xae0 [nfsv4]
> > [ 1693.689630]  nfs4_init_client+0x203/0x400 [nfsv4]
> > [ 1693.691468]  ? __pfx_nfs4_init_client+0x10/0x10 [nfsv4]
> > [ 1693.693502]  ? _raw_spin_lock_irqsave+0x98/0xf0
> > [ 1693.695141]  nfs4_set_client+0x2f4/0x520 [nfsv4]
> > [ 1693.696967]  ? __pfx_nfs4_set_client+0x10/0x10 [nfsv4]
> > [ 1693.699230]  nfs4_create_server+0x5f2/0xef0 [nfsv4]
> > [ 1693.701357]  ? _raw_spin_lock+0x85/0xe0
> > [ 1693.702758]  ? __pfx__raw_spin_lock+0x10/0x10
> > [ 1693.704344]  ? nfs_get_tree+0x61f/0x16a0 [nfs]
> > [ 1693.706160]  ? __pfx_nfs4_create_server+0x10/0x10 [nfsv4]
> > [ 1693.707376]  ? __module_get+0x26/0xc0
> > [ 1693.708061]  nfs4_try_get_tree+0xcd/0x250 [nfsv4]
> > [ 1693.708893]  vfs_get_tree+0x83/0x2d0
> > [ 1693.709534]  path_mount+0x88d/0x19a0
> > [ 1693.710100]  ? __pfx_path_mount+0x10/0x10
> > [ 1693.710718]  ? user_path_at+0xa4/0xe0
> > [ 1693.711303]  ? kmem_cache_free+0x143/0x3e0
> > [ 1693.711936]  __x64_sys_mount+0x1fb/0x270
> > [ 1693.712606]  ? __pfx___x64_sys_mount+0x10/0x10
> > [ 1693.713315]  do_syscall_64+0x4b/0x110
> > [ 1693.713889]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > [ 1693.714660] RIP: 0033:0x7f05050418fe
> > [ 1693.715233] RSP: 002b:00007fff4e0f5728 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> > [ 1693.716417] RAX: ffffffffffffffda RBX: 00007fff4e0f5900 RCX: 00007f05050418fe
> > [ 1693.717492] RDX: 00005559cea3ea70 RSI: 00005559cea3fe60 RDI: 00005559cea40140
> > [ 1693.718558] RBP: 00007fff4e0f5760 R08: 00005559cea41e60 R09: 00007f050510cb20
> > [ 1693.719620] R10: 0000000000000000 R11: 0000000000000246 R12: 00005559cea41e60
> > [ 1693.720735] R13: 00007fff4e0f5900 R14: 00005559cea41df0 R15: 0000000000000004
> > 
> > 
> > Looking at faddr2line:
> > 
> > $ ./scripts/faddr2line --list net/sunrpc/sunrpc.ko svc_set_num_threads+0xbc8/0xf80
> > svc_set_num_threads+0xbc8/0xf80:
> > 
> > svc_start_kthreads at /home/jlayton/git/kdevops/linux/net/sunrpc/svc.c:822 (discriminator 17)
> > 817 
> > 818                    svc_sock_update_bufs(serv);
> > 819                    wake_up_process(task);
> > 820 
> > 821                    /* load_acquire ensures we get value stored in svc_thread_init_status() */
> >> 822<                   wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
> > 823                    err = rqstp->rq_err;
> > 824                    if (err) {
> > 825                            svc_exit_thread(rqstp);
> > 826                            return err;
> > 827                    }
> > 
> > (inlined by) svc_set_num_threads at /home/jlayton/git/kdevops/linux/net/sunrpc/svc.c:877 (discriminator 17)
> > 872                    nrservs -= serv->sv_nrthreads;
> > 873            else
> > 874                    nrservs -= pool->sp_nrthreads;
> > 875 
> > 876            if (nrservs > 0)
> >> 877<                   return svc_start_kthreads(serv, pool, nrservs);
> > 878            if (nrservs < 0)
> > 879                    return svc_stop_kthreads(serv, pool, nrservs);
> > 880            return 0;
> > 881    }
> > 882    EXPORT_SYMBOL_GPL(svc_set_num_threads);
> > 
> > It looks like the callback thread started up properly and is past the svc_thread_init_status call.
> > 
> > $ sudo cat /proc/13246/stack
> > [<0>] svc_recv+0xcef/0x2020 [sunrpc]
> > [<0>] nfs4_callback_svc+0xb0/0x140 [nfsv4]
> > [<0>] kthread+0x29b/0x360
> > [<0>] ret_from_fork+0x30/0x70
> > [<0>] ret_from_fork_asm+0x1a/0x30
> > 
> > ...so the status should have been updated properly. Barrier problem?
> 
> Speaking as NFSD release manager: I don't intend to send the
> NFSD v6.12 PR until week of September 23rd.
> 
> I will either drop these patches before sending my PR, or I
> can squash in a fix if one is found.

When I wrote that patch I thought I understood the barriers needed for
wake/wait but I was wrong.  Quite wrong.

I do now (I believe) and I've been trying to get improved interfaces
upstream so we don't need explicit memory barrier, but no success yet.
I've posted a patch which should fix this problem.

Thanks,
BeilBrown

Re: [PATCH 0/2 v2] nfsd: fix handling of error from unshare_fs_struct()

[Chuck Lever]

On Tue, Jul 30, 2024 at 07:19:40AM +1000, NeilBrown wrote:
> This v2 applies correctly to  git/cel/linux#master.
> 
> These two patches replace my previous patch:
>  [PATCH 07/14] Change unshare_fs_struct() to never fail.
> 
> I had explored ways to change kthread_create() to avoid the need for
> GFP_NOFAIL and concluded that we can do everything we need in the sunrpc
> layer.  So the first patch here is a simple cleanup, and the second adds
> simple infrastructure for an svc thread to confirm that it has started
> up and to report if it was successful in that.
> 
> Thanks,
> NeilBrown
> 
>  [PATCH 1/2] sunrpc: merge svc_rqst_alloc() into svc_prepare_thread()
>  [PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

Applied to nfsd-next (for 6.12) and pushed. Thanks!

-- 
Chuck Lever

[PATCH 0/2] nfsd: fix handling of error from unshare_fs_struct()

[NeilBrown]

These two patches replace my previous patch:
 [PATCH 07/14] Change unshare_fs_struct() to never fail.

I had explored ways to change kthread_create() to avoid the need for
GFP_NOFAIL and concluded that we can do everything we need in the sunrpc
layer.  So the first patch here is a simple cleanup, and the second adds
simple infrastructure for an svc thread to confirm that it has started
up and to report if it was successful in that.

Thanks,
NeilBrown



 [PATCH 1/2] sunrpc: merge svc_rqst_alloc() into svc_prepare_thread()
 [PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[PATCH 2/2] sunrpc: allow svc threads to fail initialisation cleanly

[NeilBrown]

If an svc thread needs to perform some initialisation that might fail,
it has no good way to handle the failure.

Before the thread can exit it must call svc_exit_thread(), but that
requires the service mutex to be held.  The thread cannot simply take
the mutex as that could deadlock if there is a concurrent attempt to
shut down all threads (which is unlikely, but not impossible).

nfsd currently call svc_exit_thread() unprotected in the unlikely event
that unshare_fs_struct() fails.

We can clean this up by introducing svc_thread_init_status() by which an
svc thread can report whether initialisation has succeeded.  If it has,
it continues normally into the action loop.  If it has not,
svc_thread_init_status() immediately aborts the thread.
svc_start_kthread() waits for either of these to happen, and calls
svc_exit_thread() (under the mutex) if the thread aborted.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/lockd/svc.c             |  2 ++
 fs/nfs/callback.c          |  2 ++
 fs/nfsd/nfssvc.c           |  9 +++------
 include/linux/sunrpc/svc.h | 28 ++++++++++++++++++++++++++++
 net/sunrpc/svc.c           | 11 +++++++++++
 5 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index ab8042a5b895..7d74858e1abe 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -125,6 +125,8 @@ lockd(void *vrqstp)
 	struct net *net = &init_net;
 	struct lockd_net *ln = net_generic(net, lockd_net_id);
 
+	svc_thread_init_status(rqstp, 0);
+
 	/* try_to_freeze() is called from svc_recv() */
 	set_freezable();
 
diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
index 8adfcd4c8c1a..6cf92498a5ac 100644
--- a/fs/nfs/callback.c
+++ b/fs/nfs/callback.c
@@ -76,6 +76,8 @@ nfs4_callback_svc(void *vrqstp)
 {
 	struct svc_rqst *rqstp = vrqstp;
 
+	svc_thread_init_status(rqstp, 0);
+
 	set_freezable();
 
 	while (!svc_thread_should_stop(rqstp))
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 89d7918de7b1..2a99787d6a86 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -915,11 +915,9 @@ nfsd(void *vrqstp)
 
 	/* At this point, the thread shares current->fs
 	 * with the init process. We need to create files with the
-	 * umask as defined by the client instead of init's umask. */
-	if (unshare_fs_struct() < 0) {
-		printk("Unable to start nfsd thread: out of memory\n");
-		goto out;
-	}
+	 * umask as defined by the client instead of init's umask.
+	 */
+	svc_thread_init_status(rqstp, unshare_fs_struct());
 
 	current->fs->umask = 0;
 
@@ -941,7 +939,6 @@ nfsd(void *vrqstp)
 
 	atomic_dec(&nfsd_th_cnt);
 
-out:
 	/* Release the thread */
 	svc_exit_thread(rqstp);
 	return 0;
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 01383ea077ad..2c4366afc9bd 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -21,6 +21,7 @@
 #include <linux/wait.h>
 #include <linux/mm.h>
 #include <linux/pagevec.h>
+#include <linux/kthread.h>
 
 /*
  *
@@ -231,6 +232,11 @@ struct svc_rqst {
 	struct net		*rq_bc_net;	/* pointer to backchannel's
 						 * net namespace
 						 */
+
+	int			rq_err;		/* Thread sets this to inidicate
+						 * initialisation success.
+						 */
+
 	unsigned long	bc_to_initval;
 	unsigned int	bc_to_retries;
 	void **			rq_lease_breaker; /* The v4 client breaking a lease */
@@ -304,6 +310,28 @@ static inline bool svc_thread_should_stop(struct svc_rqst *rqstp)
 	return test_bit(RQ_VICTIM, &rqstp->rq_flags);
 }
 
+/**
+ * svc_thread_init_status - report whether thread has initialised successfully
+ * @rqstp: the thread in question
+ * @err: errno code
+ *
+ * After performing any initialisation that could fail, and before starting
+ * normal work, each sunrpc svc_thread must call svc_thread_init_status()
+ * with an appropriate error, or zero.
+ *
+ * If zero is passed, the thread is ready and must continue until
+ * svc_thread_should_stop() returns true.  If a non-zero error is passed
+ * the call will not return - the thread will exit.
+ */
+static inline void svc_thread_init_status(struct svc_rqst *rqstp, int err)
+{
+	/* store_release ensures svc_start_kthreads() sees the error */
+	smp_store_release(&rqstp->rq_err, err);
+	wake_up_var(&rqstp->rq_err);
+	if (err)
+		kthread_exit(1);
+}
+
 struct svc_deferred_req {
 	u32			prot;	/* protocol (UDP or TCP) */
 	struct svc_xprt		*xprt;
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index c0e91bea72e7..0fc9c97ae419 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -662,6 +662,8 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
 	if (!svc_init_buffer(rqstp, serv->sv_max_mesg, node))
 		goto out_enomem;
 
+	rqstp->rq_err = -EAGAIN; /* No error yet */
+
 	spin_lock_bh(&serv->sv_lock);
 	serv->sv_nrthreads += 1;
 	spin_unlock_bh(&serv->sv_lock);
@@ -760,6 +762,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 	struct svc_pool *chosen_pool;
 	unsigned int state = serv->sv_nrthreads-1;
 	int node;
+	int err;
 
 	do {
 		nrservs--;
@@ -782,6 +785,14 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 
 		svc_sock_update_bufs(serv);
 		wake_up_process(task);
+
+		/* load_acquire ensures we get value stored in svc_thread_init_status() */
+		wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) != -EAGAIN);
+		err = rqstp->rq_err;
+		if (err) {
+			svc_exit_thread(rqstp);
+			return err;
+		}
 	} while (nrservs > 0);
 
 	return 0;
-- 
2.44.0