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autofs4: avoid taking fs_lock during rcu-walk 

->fs_lock protects AUTOFS_INF_EXPIRING.  We need to be sure that once
the flag is set, no new references beneath the dentry are taken.  So
rcu-walk currently needs to take fs_lock before checking the flag.  This
hurts performance.

Change the expiry to a two-stage process.  First set AUTOFS_INF_NO_RCU
which forces any path walk into ref-walk mode, then drop the lock and
call synchronize_rcu().  Once that returns we can be sure no rcu-walk is
active beneath the dentry and we can check reference counts again.

Now during an RCU-walk we can test AUTOFS_INF_EXPIRING without taking
the lock as along as we test AUTOFS_INF_NO_RCU too.  If either are set,
we must abort the RCU-walk If neither are set, we know that refcounts
will be tested again after we finish the RCU-walk so we are safe to
continue.

->fs_lock is still taken in d_manage() to check for a non-trap
directory.  That will be resolved in the next patch.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Ian Kent <raven@themaw.net>
Tested-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
NeilBrown 2014-10-13 15:52:20 -07:00 committed by Linus Torvalds
parent 6ece08e618
commit 4d885f90e3
2 changed files with 42 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
fs/autofs4/autofs_i.h
View File

@ -79,6 +79,10 @@ struct autofs_info {
};
#define AUTOFS_INF_EXPIRING (1<<0) /* dentry is in the process of expiring */
#define AUTOFS_INF_NO_RCU (1<<1) /* the dentry is being considered
* for expiry, so RCU_walk is
* not permitted
*/
#define AUTOFS_INF_PENDING (1<<2) /* dentry pending mount */
struct autofs_wait_queue {

46
fs/autofs4/expire.c
View File

@ -321,10 +321,19 @@ struct dentry *autofs4_expire_direct(struct super_block *sb,
if (ino->flags & AUTOFS_INF_PENDING)
goto out;
if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
ino->flags |= AUTOFS_INF_EXPIRING;
init_completion(&ino->expire_complete);
ino->flags |= AUTOFS_INF_NO_RCU;
spin_unlock(&sbi->fs_lock);
return root;
synchronize_rcu();
spin_lock(&sbi->fs_lock);
if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
ino->flags |= AUTOFS_INF_EXPIRING;
smp_mb();
ino->flags &= ~AUTOFS_INF_NO_RCU;
init_completion(&ino->expire_complete);
spin_unlock(&sbi->fs_lock);
return root;
}
ino->flags &= ~AUTOFS_INF_NO_RCU;
}
out:
spin_unlock(&sbi->fs_lock);
@ -442,12 +451,29 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
dentry = NULL;
while ((dentry = get_next_positive_subdir(dentry, root))) {
spin_lock(&sbi->fs_lock);
expired = should_expire(dentry, mnt, timeout, how);
if (expired) {
ino = autofs4_dentry_ino(dentry);
if (ino->flags & AUTOFS_INF_NO_RCU)
expired = NULL;
else
expired = should_expire(dentry, mnt, timeout, how);
if (!expired) {
spin_unlock(&sbi->fs_lock);
continue;
}
ino = autofs4_dentry_ino(expired);
ino->flags |= AUTOFS_INF_NO_RCU;
spin_unlock(&sbi->fs_lock);
synchronize_rcu();
spin_lock(&sbi->fs_lock);
if (should_expire(expired, mnt, timeout, how)) {
if (expired != dentry)
dput(dentry);
goto found;
}
ino->flags &= ~AUTOFS_INF_NO_RCU;
if (expired != dentry)
dput(expired);
spin_unlock(&sbi->fs_lock);
}
return NULL;
@ -455,8 +481,9 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
found:
DPRINTK("returning %p %.*s",
expired, (int)expired->d_name.len, expired->d_name.name);
ino = autofs4_dentry_ino(expired);
ino->flags |= AUTOFS_INF_EXPIRING;
smp_mb();
ino->flags &= ~AUTOFS_INF_NO_RCU;
init_completion(&ino->expire_complete);
spin_unlock(&sbi->fs_lock);
spin_lock(&sbi->lookup_lock);
@ -476,11 +503,14 @@ int autofs4_expire_wait(struct dentry *dentry, int rcu_walk)
int status;
/* Block on any pending expire */
if (!(ino->flags & (AUTOFS_INF_EXPIRING | AUTOFS_INF_NO_RCU)))
return 0;
if (rcu_walk)
return -ECHILD;
spin_lock(&sbi->fs_lock);
if (ino->flags & AUTOFS_INF_EXPIRING) {
spin_unlock(&sbi->fs_lock);
if (rcu_walk)
return -ECHILD;
DPRINTK("waiting for expire %p name=%.*s",
dentry, dentry->d_name.len, dentry->d_name.name);