From: Julia Lawall <[email protected]>
list_for_each_entry uses its first argument to move from one element to the
next, so modifying it can break the iteration.
The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)
// <smpl>
@r@
iterator name list_for_each_entry;
expression x,E;
position p1,p2;
@@
list_for_each_entry@p1(x,...) { <... x =@p2 E ...> }
@@
expression x,E;
position r.p1,r.p2;
statement S;
@@
*x =@p2 E
...
list_for_each_entry@p1(x,...) S
// </smpl>
Signed-off-by: Julia Lawall <[email protected]>
---
I don't know whether this is the right solution, but it seems plausible
considering the subsequent test on lock. In any case, setting lock to NULL
and then going back to the top of the loop does not work.
fs/ocfs2/dlm/dlmrecovery.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/ocfs2/dlm/dlmrecovery.c b/fs/ocfs2/dlm/dlmrecovery.c
index 9dfaac7..7084a11 100644
--- a/fs/ocfs2/dlm/dlmrecovery.c
+++ b/fs/ocfs2/dlm/dlmrecovery.c
@@ -1792,10 +1792,10 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
tmpq = dlm_list_idx_to_ptr(res, j);
list_for_each_entry(lock, tmpq, list) {
- if (lock->ml.cookie != ml->cookie)
+ if (lock->ml.cookie != ml->cookie) {
lock = NULL;
- else
break;
+ }
}
if (lock)
break;
On Sat, Aug 07, 2010 at 11:09:13AM +0200, Julia Lawall wrote:
> From: Julia Lawall <[email protected]>
>
> list_for_each_entry uses its first argument to move from one element to the
> next, so modifying it can break the iteration.
Thanks for catching the bug. It was introduced by 800deef3
[ocfs2: use list_for_each_entry where benefical]. I blame Christoph.
> diff --git a/fs/ocfs2/dlm/dlmrecovery.c b/fs/ocfs2/dlm/dlmrecovery.c
> index 9dfaac7..7084a11 100644
> --- a/fs/ocfs2/dlm/dlmrecovery.c
> +++ b/fs/ocfs2/dlm/dlmrecovery.c
> @@ -1792,10 +1792,10 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
> for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
> tmpq = dlm_list_idx_to_ptr(res, j);
> list_for_each_entry(lock, tmpq, list) {
> - if (lock->ml.cookie != ml->cookie)
> + if (lock->ml.cookie != ml->cookie) {
> lock = NULL;
> - else
> break;
> + }
> }
> if (lock)
> break;
However, this is not the correct solution. The goal of the
original code, which used to use list_for_each(), was to leave lock
non-NULL if the cookie was found. Your version merely exits the loop on
the first non-matching entry, always leaving lock==NULL if there is a
non-matching entry.
One possible solution is to return the original code:
--8<-----------------------------------------------------------------
@@ -1747,7 +1747,7 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
struct dlm_migratable_lockres *mres)
{
struct dlm_migratable_lock *ml;
- struct list_head *queue;
+ struct list_head *queue, *iter;
struct list_head *tmpq = NULL;
struct dlm_lock *newlock = NULL;
struct dlm_lockstatus *lksb = NULL;
@@ -1791,11 +1791,12 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
spin_lock(&res->spinlock);
for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
tmpq = dlm_list_idx_to_ptr(res, j);
- list_for_each_entry(lock, tmpq, list) {
- if (lock->ml.cookie != ml->cookie)
- lock = NULL;
- else
+ list_for_each(iter, tmpq) {
+ lock = list_entry(iter, struct dlm_lock, list);
+
+ if (lock->ml.cookie == ml->cookie)
break;
+ lock = NULL;
}
if (lock)
break;
-->8-----------------------------------------------------------------
Another approach would be to keep list_for_each_entry() around,
but use a better check for entry existence:
--8<-----------------------------------------------------------------
@@ -1792,13 +1792,12 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
tmpq = dlm_list_idx_to_ptr(res, j);
list_for_each_entry(lock, tmpq, list) {
- if (lock->ml.cookie != ml->cookie)
- lock = NULL;
- else
+ if (lock->ml.cookie == ml->cookie)
break;
}
- if (lock)
+ if (&lock->list != tmpq)
break;
+ lock = NULL;
}
/* lock is always created locally first, and
-->8-----------------------------------------------------------------
I think I like the second one better. Sunil, what do you think?
Joel
--
Life's Little Instruction Book #335
"Every so often, push your luck."
Joel Becker
Consulting Software Developer
Oracle
E-mail: [email protected]
Phone: (650) 506-8127
On Wed, 11 Aug 2010, Joel Becker wrote:
> On Sat, Aug 07, 2010 at 11:09:13AM +0200, Julia Lawall wrote:
> > From: Julia Lawall <[email protected]>
> >
> > list_for_each_entry uses its first argument to move from one element to the
> > next, so modifying it can break the iteration.
>
> Thanks for catching the bug. It was introduced by 800deef3
> [ocfs2: use list_for_each_entry where benefical]. I blame Christoph.
>
> > diff --git a/fs/ocfs2/dlm/dlmrecovery.c b/fs/ocfs2/dlm/dlmrecovery.c
> > index 9dfaac7..7084a11 100644
> > --- a/fs/ocfs2/dlm/dlmrecovery.c
> > +++ b/fs/ocfs2/dlm/dlmrecovery.c
> > @@ -1792,10 +1792,10 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
> > for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
> > tmpq = dlm_list_idx_to_ptr(res, j);
> > list_for_each_entry(lock, tmpq, list) {
> > - if (lock->ml.cookie != ml->cookie)
> > + if (lock->ml.cookie != ml->cookie) {
> > lock = NULL;
> > - else
> > break;
> > + }
> > }
> > if (lock)
> > break;
>
> However, this is not the correct solution. The goal of the
> original code, which used to use list_for_each(), was to leave lock
> non-NULL if the cookie was found. Your version merely exits the loop on
> the first non-matching entry, always leaving lock==NULL if there is a
> non-matching entry.
> One possible solution is to return the original code:
>
> --8<-----------------------------------------------------------------
> @@ -1747,7 +1747,7 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
> struct dlm_migratable_lockres *mres)
> {
> struct dlm_migratable_lock *ml;
> - struct list_head *queue;
> + struct list_head *queue, *iter;
> struct list_head *tmpq = NULL;
> struct dlm_lock *newlock = NULL;
> struct dlm_lockstatus *lksb = NULL;
> @@ -1791,11 +1791,12 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
> spin_lock(&res->spinlock);
> for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
> tmpq = dlm_list_idx_to_ptr(res, j);
> - list_for_each_entry(lock, tmpq, list) {
> - if (lock->ml.cookie != ml->cookie)
> - lock = NULL;
> - else
> + list_for_each(iter, tmpq) {
> + lock = list_entry(iter, struct dlm_lock, list);
> +
> + if (lock->ml.cookie == ml->cookie)
> break;
> + lock = NULL;
> }
> if (lock)
> break;
> -->8-----------------------------------------------------------------
>
> Another approach would be to keep list_for_each_entry() around,
> but use a better check for entry existence:
>
> --8<-----------------------------------------------------------------
> @@ -1792,13 +1792,12 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
> for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
> tmpq = dlm_list_idx_to_ptr(res, j);
> list_for_each_entry(lock, tmpq, list) {
> - if (lock->ml.cookie != ml->cookie)
> - lock = NULL;
> - else
> + if (lock->ml.cookie == ml->cookie)
> break;
> }
> - if (lock)
> + if (&lock->list != tmpq)
> break;
> + lock = NULL;
> }
This seems a bit ugly to me, since it exposes the implementation of the
list abstraction. What about the following:
lock = NULL;
list_for_each_entry(x, tmpq, list) {
if (x->ml.cookie == ml->cookie) {
lock = x;
break;
}
}
julia
On Thu, Aug 12, 2010 at 07:46:13AM +0200, Julia Lawall wrote:
>
> This seems a bit ugly to me, since it exposes the implementation of the
> list abstraction. What about the following:
>
> lock = NULL;
> list_for_each_entry(x, tmpq, list) {
> if (x->ml.cookie == ml->cookie) {
> lock = x;
> break;
> }
> }
>
I agree with you that it's ugly as pants. Maybe someone could write an
at_list_start() macro?
regards,
dan carpenter
On Thu, Aug 12, 2010 at 07:46:13AM +0200, Julia Lawall wrote:
> This seems a bit ugly to me, since it exposes the implementation of the
> list abstraction. What about the following:
Actually, we're going to go back to the original list_for_each()
and list_entry() version. It's a well understood idiom.
We're also going to re-audit the rest of the patch that changed
list_for_each() to list_for_each_entry(), just in case we missed another
semantic error.
Joel
--
Life's Little Instruction Book #30
"Never buy a house without a fireplace."
Joel Becker
Consulting Software Developer
Oracle
E-mail: [email protected]
Phone: (650) 506-8127