Messages in this thread Patch in this message | | | From | Paul Menage <> | Subject | [PATCH] Avoid unnecessary d_count/mnt_count dirtying in link_path_walk() | Date | Tue, 16 Jul 2002 18:27:04 -0700 |
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On the error return path (including -ENOENT) in link_path_walk(), we call
unlock_nd(nd); path_release(nd);
This is equivalent to
mntget(nd->mnt); dget_locked(nd->dentry); ... spin_unlock(&dcache_lock); ... dput(nd->dentry); mntput(nd->mnt);
The only way that this can have an effect other than bouncing the d_count and mnt_count cachelines of the last resolved component is if the dentry d_count reaches 0, and was either unhashed or had a d_delete() method that returned true (in which case the dentry is thrown away).
- Since no-one else can decrement the usage count to 0 while we hold dcache_lock (due to the use of atomic_dec_and_lock() in dput()) this can only occur if the usage count is zero originally (i.e. before we called unlock_nd()).
- Since no-one else can unhash the dentry without dcache_lock (which we hold) the dentry must be hashed if its reference count was originally 0.
- if dentry->d_op->d_delete() exists, then plausibly it could return 1 to request that the dentry by unhashed. But this failed lookup has no noticable side-effects (other than perhaps setting DCACHE_REFERENCED) and might easily have not occurred (and hence d_delete() not called). So we can't lose correctness by not calling d_delete(), (unless the d_delete() method would have spuriously decided to kill the dentry purely due to it being marked DCACHE_REFERENCED, but that would be pretty perverse).
Hence it is safe to skip the dget_locked()/dput().
The original pointer to nd->mnt came either from
- the current root/pwd (protected by dcache_lock)
- following a mount (protected by dcache_lock)
- was in nd->mnt when we called lock_nd() (and will be released when we call mntput() on nd->old_mnt.
Hence it is safe to skip the mntget()/mntput()
This patch adds release_locked_nd(), which skips the dget_locked() and mntget(), and makes use of it in the error .
Paul
diff -X /mnt/elbrus/home/pmenage/dontdiff -Naur linux-2.5.26/fs/namei.c linux-2.5.26-unlocknd/fs/namei.c --- linux-2.5.26/fs/namei.c Tue Jul 16 16:49:31 2002 +++ linux-2.5.26-unlocknd/fs/namei.c Tue Jul 16 17:59:10 2002 @@ -275,17 +275,22 @@ } /*for fastwalking*/ -static inline void unlock_nd(struct nameidata *nd) +static inline void release_locked_nd(struct nameidata *nd) { struct vfsmount *mnt = nd->old_mnt; struct dentry *dentry = nd->old_dentry; - mntget(nd->mnt); - dget_locked(nd->dentry); nd->old_mnt = NULL; nd->old_dentry = NULL; spin_unlock(&dcache_lock); dput(dentry); mntput(mnt); +} + +static inline void unlock_nd(struct nameidata *nd) +{ + mntget(nd->mnt); + dget_locked(nd->dentry); + release_locked_nd(nd); } static inline void lock_nd(struct nameidata *nd) @@ -737,8 +742,8 @@ mntput(pinned.mnt); return 0; } - unlock_nd(nd); - path_release(nd); + + release_locked_nd(nd); return_err: dput(pinned.dentry); mntput(pinned.mnt);
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