On Mon, Sep 09, 2013 at 07:06:04PM +0100, Al Viro wrote:
> I like my proposal for the set of primitives better:
>
> static inline bool seqretry_and_lock(seqlock_t *lock, unsigned *seq):
> {
> if ((*seq & 1) || !read_seqretry(lock, *seq))
> return true;
> *seq |= 1;
> write_seqlock(lock);
> return false;
> }
>
> static inline void seqretry_done(seqlock_t *lock, unsigned seq)
> {
> if (seq & 1)
> write_sequnlock(lock);
> }
>
> with the prepend_path() and friends becoming
>
> rcu_read_lock();
> seq = read_seqbegin(&rename_lock);
> again:
> ....
> if (!seqretry_and_lock(&rename_lock, seq))
> goto again; /* now as writer */
> seqretry_done(&rename_lock, seq);
> rcu_read_unlock();
Actually, it's better for prepend_path() as well, because it's actually
rcu_read_lock();
seq = read_seqbegin(&rename_lock);
again:
....
if (error)
goto done;
....
if (!seqretry_and_lock(&rename_lock, seq))
goto again; /* now as writer */
done:
seqretry_done(&rename_lock, seq);
rcu_read_unlock();
Posted variant will sometimes hit the following path:
* seq_readlock()
* start generating the output
* hit an error
[another process has taken and released rename_lock for some reason]
* hit read_seqretry_and_unlock(), which returns 1.
* retry everything with seq_writelock(), despite the error.
It's not too horrible (we won't be looping indefinitely, ignoring error
all along), but it's certainly subtle enough...
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