On Wed, Nov 01, 2023 at 06:20:58AM +0000, Al Viro wrote:
> Calls of retain_dentry() happen immediately after getting false
> from fast_dput() and getting true from retain_dentry() is
> treated the same way as non-zero refcount would be treated by
> fast_dput() - unlock dentry and bugger off.
>
> Doing that in fast_dput() itself is simpler.
FWIW, I wonder if it would be better to reorganize it a bit -
// in some cases we can show that retain_dentry() would return true
// without having to take ->d_lock
< your comments regarding that part go here>
static inline bool lockless_retain_dentry(struct dentry *dentry)
{
unsigned int d_flags;
smp_rmb();
d_flags = READ_ONCE(dentry->d_flags);
d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_OP_DELETE |
DCACHE_DISCONNECTED | DCACHE_DONTCACHE;
/* Nothing to do? Dropping the reference was all we needed? */
if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))
return true;
return false;
}
and fast_dput() becomes
{
int ret;
// try to do decrement locklessly
ret = lockref_put_return(&dentry->d_lockref);
if (likely(ret >= 0)) {
// could we show that full check would succeed?
if (ret || lockless_retain_dentry(dentry))
return true;
// too bad, have to lock it and do full variant
spin_lock(&dentry->d_lock);
} else {
// failed, no chance to avoid grabbing lock
spin_lock(&dentry->d_lock);
// underflow? whine and back off - we are done
if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) {
spin_unlock(&dentry->d_lock);
return true;
}
// decrement it under lock, then...
dentry->d_lockref.count--;
}
// full check it is...
if (dentry->d_lockref.count || retain_dentry(dentry)) {
spin_unlock(&dentry->d_lock);
return true;
}
return false;
}
Might be easier to follow that way... Another thing: would you mind
#if USE_CMPXCHG_LOCKREF
extern int lockref_put_return(struct lockref *);
#else
static inline int lockref_put_return(struct lockref *l)
{
return -1;
}
#endif
in include/linux/lockref.h? Would be useful on DEBUG_SPINLOCK configs...
