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SubjectRe: [PATCH - take 2] knfsd: nfsd: Handle ERESTARTSYS from syscalls. (and possible kthread_stop changes)
On Sunday June 22, jlayton@redhat.com wrote:
> On Mon, 23 Jun 2008 10:20:59 +1000
> Neil Brown <neilb@suse.de> wrote:
> >
> > I think my leaning is just to remove the distinction. About the worst
> > that can happen if the filesystem decides to respond to the signal is
> > that you get a short write or a short read, both of which should be
> > correctly handled by the client.
> >
>
> Good points all around. If the sigmask juggling makes no sense, then
> I'm OK with removing it. I'd prefer that we simplify the code rather
> than trying to get clever with init scripts anyway...
>

Let's do it?

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 941041f..914e579 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -36,12 +36,7 @@

#define NFSDDBG_FACILITY NFSDDBG_SVC

-/* these signals will be delivered to an nfsd thread
- * when handling a request
- */
-#define ALLOWED_SIGS (sigmask(SIGKILL))
-/* these signals will be delivered to an nfsd thread
- * when not handling a request. i.e. when waiting
+/* These signals can be used to shutdown an nfsd thread.
*/
#define SHUTDOWN_SIGS (sigmask(SIGKILL) | sigmask(SIGHUP) | sigmask(SIGINT) | sigmask(SIGQUIT))
/* if the last thread dies with SIGHUP, then the exports table is
@@ -396,7 +391,7 @@ nfsd(struct svc_rqst *rqstp)
{
struct fs_struct *fsp;
int err;
- sigset_t shutdown_mask, allowed_mask;
+ sigset_t shutdown_mask;

/* Lock module and set up kernel thread */
lock_kernel();
@@ -415,7 +410,8 @@ nfsd(struct svc_rqst *rqstp)
current->fs->umask = 0;

siginitsetinv(&shutdown_mask, SHUTDOWN_SIGS);
- siginitsetinv(&allowed_mask, ALLOWED_SIGS);
+ /* Block all but the shutdown signals */
+ sigprocmask(SIG_SETMASK, &shutdown_mask, NULL);

nfsdstats.th_cnt++;

@@ -435,8 +431,6 @@ nfsd(struct svc_rqst *rqstp)
* The main request loop
*/
for (;;) {
- /* Block all but the shutdown signals */
- sigprocmask(SIG_SETMASK, &shutdown_mask, NULL);

/*
* Find a socket with data available and call its
@@ -452,9 +446,6 @@ nfsd(struct svc_rqst *rqstp)
/* Lock the export hash tables for reading. */
exp_readlock();

- /* Process request with signals blocked. */
- sigprocmask(SIG_SETMASK, &allowed_mask, NULL);
-
svc_process(rqstp);

/* Unlock export hash tables */

>
> The latest patch doesn't use kthread_stop with nfsd. I figured it was
> best to avoid having multiple takedown methods, and since we're using
> signals here anyway, I just kept signaling as the only way to take down
> nfsd. Plus, as you mention -- all kthread_stop's are serialized, which
> could have meant that nfsd's would take a long time to come down on a
> busy box with a lot of them.

Oh, good. I must have been looking at an old patch.

>
> As a side note, I'm not terribly thrilled with how kthread_stop is
> implemented. I worry that a stuck kthread would block unrelated
> kthreads from coming down. I did a patch 6 mos ago or so to make it so
> that kthread_stop's could be done in parallel. The downside was that it
> added a new field to the task_struct (which is already too bloated,
> IMO). It might be worth resurrecting that at some point (possibly
> making the new field a union with another field that's unused in
> kthreads), or considering a different approach to parallelize
> kthread_stop.

One the one hand, kthreads are expected to not block and to check for
kthread_should_stop very often, so this wouldn't be a problem.
On the other hand, expectations like this are quickly invalidated, and
the code probably should be fixed.
I suspect we could do without adding anything to task_struct.
Instead of just one kthread_stop_info, have a linked list, one entry
for each thread being stopped at the moment. kthread_stop adds to
the list, sets PF_EXITING (I hope that is safe) and wakes the process.
kthread_should_stop tests PF_EXITING.
When kthread() finishes, it does a linear search (the list should be
short) and calls complete and the relevant .done.

Something like this?

NeilBrown


diff --git a/kernel/kthread.c b/kernel/kthread.c
index bd1b9ea..107290a 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -39,12 +39,13 @@ struct kthread_stop_info
struct task_struct *k;
int err;
struct completion done;
+ struct kthread_stp_info *next;
};

/* Thread stopping is done by setthing this var: lock serializes
* multiple kthread_stop calls. */
static DEFINE_MUTEX(kthread_stop_lock);
-static struct kthread_stop_info kthread_stop_info;
+static struct kthread_stop_info *kthread_stop_info;

/**
* kthread_should_stop - should this kthread return now?
@@ -55,7 +56,7 @@ static struct kthread_stop_info kthread_stop_info;
*/
int kthread_should_stop(void)
{
- return (kthread_stop_info.k == current);
+ return test_bit(PF_EXITING, &current.flags);
}
EXPORT_SYMBOL(kthread_should_stop);

@@ -80,8 +81,17 @@ static int kthread(void *_create)

/* It might have exited on its own, w/o kthread_stop. Check. */
if (kthread_should_stop()) {
- kthread_stop_info.err = ret;
- complete(&kthread_stop_info.done);
+ struct kthread_stop_info **ksip;
+ mutex_lock(&kthread_stop_lock);
+ for (ksip = &kthread_stop_info ; *ksip ; ksip = &(*ksip)->next)
+ if ((*ksip)->k == current)
+ break;
+ *ksip = (*ksip)->next;
+ (*ksip)->next = NULL;
+ mutex_unlock(&kthread_stop_lock);
+
+ ksi->err = ret;
+ complete(&ksi->done);
}
return 0;
}
@@ -199,26 +209,20 @@ EXPORT_SYMBOL(kthread_bind);
int kthread_stop(struct task_struct *k)
{
int ret;
+ struct kthread_stop_info ksi;

mutex_lock(&kthread_stop_lock);

- /* It could exit after stop_info.k set, but before wake_up_process. */
- get_task_struct(k);
+ init_completion(&ksi->done);
+ ksi->k = k;

- /* Must init completion *before* thread sees kthread_stop_info.k */
- init_completion(&kthread_stop_info.done);
- smp_wmb();
-
- /* Now set kthread_should_stop() to true, and wake it up. */
- kthread_stop_info.k = k;
+ set_bit(PF_EXITING, &k->flags);
wake_up_process(k);
- put_task_struct(k);
+ mutex_unlock(&kthread_stop_lock);

- /* Once it dies, reset stop ptr, gather result and we're done. */
- wait_for_completion(&kthread_stop_info.done);
- kthread_stop_info.k = NULL;
+ /* Once it dies gather result and we're done. */
+ wait_for_completion(&ksi->done);
ret = kthread_stop_info.err;
- mutex_unlock(&kthread_stop_lock);

return ret;
}

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