* fs/namei.c: Make status likely to be ECHILD in lookup_fast()
@ 2020-12-09 20:24 Steven Rostedt
2020-12-09 20:35 ` Al Viro
0 siblings, 1 reply; 3+ messages in thread
From: Steven Rostedt @ 2020-12-09 20:24 UTC (permalink / raw)
To: LKML; +Cc: Al Viro, linux-fsdevel
From: Steven Rostedt (VMware) <rostedt@goodmis.org>
Running my yearly branch profiling code, it detected a 100% wrong branch
condition in name.c for lookup_fast(). The code in question has:
status = d_revalidate(dentry, nd->flags);
if (likely(status > 0))
return dentry;
if (unlazy_child(nd, dentry, seq))
return ERR_PTR(-ECHILD);
if (unlikely(status == -ECHILD))
/* we'd been told to redo it in non-rcu mode */
status = d_revalidate(dentry, nd->flags);
If the status of the d_revalidate() is greater than zero, then the function
finishes. Otherwise, if it is an "unlazy_child" it returns with -ECHILD.
After the above two checks, the status is compared to -ECHILD, as that is
what is returned if the original d_revalidate() needed to be done in a
non-rcu mode.
Especially this path is called in a condition of:
if (nd->flags & LOOKUP_RCU) {
And most of the d_revalidate() functions have:
if (flags & LOOKUP_RCU)
return -ECHILD;
It appears that that is the only case that this if statement is triggered
on two of my machines, running in production.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
diff --git a/fs/namei.c b/fs/namei.c
index d4a6dd772303..8dd734efae9b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1495,7 +1495,7 @@ static struct dentry *lookup_fast(struct nameidata *nd,
return dentry;
if (unlazy_child(nd, dentry, seq))
return ERR_PTR(-ECHILD);
- if (unlikely(status == -ECHILD))
+ if (likely(status == -ECHILD))
/* we'd been told to redo it in non-rcu mode */
status = d_revalidate(dentry, nd->flags);
} else {
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: fs/namei.c: Make status likely to be ECHILD in lookup_fast()
2020-12-09 20:24 fs/namei.c: Make status likely to be ECHILD in lookup_fast() Steven Rostedt
@ 2020-12-09 20:35 ` Al Viro
2020-12-09 20:58 ` Steven Rostedt
0 siblings, 1 reply; 3+ messages in thread
From: Al Viro @ 2020-12-09 20:35 UTC (permalink / raw)
To: Steven Rostedt; +Cc: LKML, linux-fsdevel
On Wed, Dec 09, 2020 at 03:24:03PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt (VMware) <rostedt@goodmis.org>
>
> Running my yearly branch profiling code, it detected a 100% wrong branch
> condition in name.c for lookup_fast(). The code in question has:
>
> status = d_revalidate(dentry, nd->flags);
> if (likely(status > 0))
> return dentry;
> if (unlazy_child(nd, dentry, seq))
> return ERR_PTR(-ECHILD);
> if (unlikely(status == -ECHILD))
> /* we'd been told to redo it in non-rcu mode */
> status = d_revalidate(dentry, nd->flags);
>
> If the status of the d_revalidate() is greater than zero, then the function
> finishes. Otherwise, if it is an "unlazy_child" it returns with -ECHILD.
> After the above two checks, the status is compared to -ECHILD, as that is
> what is returned if the original d_revalidate() needed to be done in a
> non-rcu mode.
>
> Especially this path is called in a condition of:
>
> if (nd->flags & LOOKUP_RCU) {
>
> And most of the d_revalidate() functions have:
>
> if (flags & LOOKUP_RCU)
> return -ECHILD;
Umm... That depends upon the filesystem mix involved; said that, I'd rather
drop that "unlikely"...
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: fs/namei.c: Make status likely to be ECHILD in lookup_fast()
2020-12-09 20:35 ` Al Viro
@ 2020-12-09 20:58 ` Steven Rostedt
0 siblings, 0 replies; 3+ messages in thread
From: Steven Rostedt @ 2020-12-09 20:58 UTC (permalink / raw)
To: Al Viro; +Cc: LKML, linux-fsdevel
On Wed, 9 Dec 2020 20:35:00 +0000
Al Viro <viro@zeniv.linux.org.uk> wrote:
> > And most of the d_revalidate() functions have:
> >
> > if (flags & LOOKUP_RCU)
> > return -ECHILD;
>
> Umm... That depends upon the filesystem mix involved; said that, I'd rather
> drop that "unlikely"...
Sure enough. I'll send a v2.
Thanks,
-- Steve
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-12-09 20:24 fs/namei.c: Make status likely to be ECHILD in lookup_fast() Steven Rostedt
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2020-12-09 20:58 ` Steven Rostedt
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