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SubjectRe: frequent lockups in 3.18rc4
On Sun, Dec 14, 2014 at 03:14:29AM +0000, Al Viro wrote:
> On Sat, Dec 13, 2014 at 05:35:17PM -0800, Linus Torvalds wrote:
> > On Sat, Dec 13, 2014 at 4:33 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > >
> > > So does SMP - this_cpu_dec() relies on preemption being disabled.
> >
> > No. really. It very much does not. Not on x86, not elsewhere. It's
> > part of the whole point of "this_cpu_p()". They are preemption and
> > interrupt safe.
> >
> > It's the "__this_cpu_op()" ones that need external protection.
>
> Right you are - I really need to get some coffee... Sorry...
>
> FWIW, do we need to disable interrupts there? After all, mnt_want_write()
> and mnt_drop_write() shouldn't be done from interrupt context - they can
> happen via schedule_delayed_work(), but that's it...

OK, having looked through the tree - we really don't need to bother with
disabling interrupts (fortunately - or UP case would be broken). So how
about turning those into __this_cpu_{inc,dec} and yes, moving preempt
disabling into mnt_{inc,dec}_writers()? Like this:

diff --git a/fs/namespace.c b/fs/namespace.c
index 5b66b2b..48cb162 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -274,20 +274,32 @@ EXPORT_SYMBOL_GPL(__mnt_is_readonly);

static inline void mnt_inc_writers(struct mount *mnt)
{
+ preempt_disable();
#ifdef CONFIG_SMP
- this_cpu_inc(mnt->mnt_pcp->mnt_writers);
+ __this_cpu_inc(mnt->mnt_pcp->mnt_writers);
#else
mnt->mnt_writers++;
#endif
+ preempt_enable();
}

-static inline void mnt_dec_writers(struct mount *mnt)
+/**
+ * __mnt_drop_write - give up write access to a mount
+ * @mnt: the mount on which to give up write access
+ *
+ * Tells the low-level filesystem that we are done
+ * performing writes to it. Must be matched with
+ * __mnt_want_write() call above.
+ */
+void __mnt_drop_write(struct vfsmount *m)
{
+ preempt_disable();
#ifdef CONFIG_SMP
- this_cpu_dec(mnt->mnt_pcp->mnt_writers);
+ __this_cpu_dec(real_mount(m)->mnt_pcp->mnt_writers);
#else
- mnt->mnt_writers--;
+ real_mount(m)->mnt_writers--;
#endif
+ preempt_enable();
}

static unsigned int mnt_get_writers(struct mount *mnt)
@@ -336,7 +348,6 @@ int __mnt_want_write(struct vfsmount *m)
struct mount *mnt = real_mount(m);
int ret = 0;

- preempt_disable();
mnt_inc_writers(mnt);
/*
* The store to mnt_inc_writers must be visible before we pass
@@ -353,10 +364,9 @@ int __mnt_want_write(struct vfsmount *m)
*/
smp_rmb();
if (mnt_is_readonly(m)) {
- mnt_dec_writers(mnt);
+ __mnt_drop_write(m);
ret = -EROFS;
}
- preempt_enable();

return ret;
}
@@ -399,9 +409,7 @@ int mnt_clone_write(struct vfsmount *mnt)
/* superblock may be r/o */
if (__mnt_is_readonly(mnt))
return -EROFS;
- preempt_disable();
mnt_inc_writers(real_mount(mnt));
- preempt_enable();
return 0;
}
EXPORT_SYMBOL_GPL(mnt_clone_write);
@@ -441,21 +449,6 @@ int mnt_want_write_file(struct file *file)
EXPORT_SYMBOL_GPL(mnt_want_write_file);

/**
- * __mnt_drop_write - give up write access to a mount
- * @mnt: the mount on which to give up write access
- *
- * Tells the low-level filesystem that we are done
- * performing writes to it. Must be matched with
- * __mnt_want_write() call above.
- */
-void __mnt_drop_write(struct vfsmount *mnt)
-{
- preempt_disable();
- mnt_dec_writers(real_mount(mnt));
- preempt_enable();
-}
-
-/**
* mnt_drop_write - give up write access to a mount
* @mnt: the mount on which to give up write access
*

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