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authorLinus Torvalds <torvalds@linux-foundation.org>2020-03-18 07:52:21 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2020-03-18 13:03:38 -0700
commitdcf23ac3e846ca0cf626c155a0e3fcbbcf4fae8a (patch)
treebcec3946a19e79751a97280bed14a633ad1a8ff8 /fs
parentmm: slub: be more careful about the double cmpxchg of freelist (diff)
downloadlinux-dev-dcf23ac3e846ca0cf626c155a0e3fcbbcf4fae8a.tar.xz
linux-dev-dcf23ac3e846ca0cf626c155a0e3fcbbcf4fae8a.zip
locks: reinstate locks_delete_block optimization
There is measurable performance impact in some synthetic tests due to commit 6d390e4b5d48 (locks: fix a potential use-after-free problem when wakeup a waiter). Fix the race condition instead by clearing the fl_blocker pointer after the wake_up, using explicit acquire/release semantics. This does mean that we can no longer use the clearing of fl_blocker as the wait condition, so switch the waiters over to checking whether the fl_blocked_member list_head is empty. Reviewed-by: yangerkun <yangerkun@huawei.com> Reviewed-by: NeilBrown <neilb@suse.de> Fixes: 6d390e4b5d48 (locks: fix a potential use-after-free problem when wakeup a waiter) Signed-off-by: Jeff Layton <jlayton@kernel.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs')
-rw-r--r--fs/cifs/file.c3
-rw-r--r--fs/locks.c54
2 files changed, 50 insertions, 7 deletions
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 3b942ecdd4be..8f9d849a0012 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -1169,7 +1169,8 @@ try_again:
rc = posix_lock_file(file, flock, NULL);
up_write(&cinode->lock_sem);
if (rc == FILE_LOCK_DEFERRED) {
- rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
+ rc = wait_event_interruptible(flock->fl_wait,
+ list_empty(&flock->fl_blocked_member));
if (!rc)
goto try_again;
locks_delete_block(flock);
diff --git a/fs/locks.c b/fs/locks.c
index 426b55d333d5..b8a31c1c4fff 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -725,7 +725,6 @@ static void __locks_delete_block(struct file_lock *waiter)
{
locks_delete_global_blocked(waiter);
list_del_init(&waiter->fl_blocked_member);
- waiter->fl_blocker = NULL;
}
static void __locks_wake_up_blocks(struct file_lock *blocker)
@@ -740,6 +739,13 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
waiter->fl_lmops->lm_notify(waiter);
else
wake_up(&waiter->fl_wait);
+
+ /*
+ * The setting of fl_blocker to NULL marks the "done"
+ * point in deleting a block. Paired with acquire at the top
+ * of locks_delete_block().
+ */
+ smp_store_release(&waiter->fl_blocker, NULL);
}
}
@@ -753,11 +759,42 @@ int locks_delete_block(struct file_lock *waiter)
{
int status = -ENOENT;
+ /*
+ * If fl_blocker is NULL, it won't be set again as this thread "owns"
+ * the lock and is the only one that might try to claim the lock.
+ *
+ * We use acquire/release to manage fl_blocker so that we can
+ * optimize away taking the blocked_lock_lock in many cases.
+ *
+ * The smp_load_acquire guarantees two things:
+ *
+ * 1/ that fl_blocked_requests can be tested locklessly. If something
+ * was recently added to that list it must have been in a locked region
+ * *before* the locked region when fl_blocker was set to NULL.
+ *
+ * 2/ that no other thread is accessing 'waiter', so it is safe to free
+ * it. __locks_wake_up_blocks is careful not to touch waiter after
+ * fl_blocker is released.
+ *
+ * If a lockless check of fl_blocker shows it to be NULL, we know that
+ * no new locks can be inserted into its fl_blocked_requests list, and
+ * can avoid doing anything further if the list is empty.
+ */
+ if (!smp_load_acquire(&waiter->fl_blocker) &&
+ list_empty(&waiter->fl_blocked_requests))
+ return status;
+
spin_lock(&blocked_lock_lock);
if (waiter->fl_blocker)
status = 0;
__locks_wake_up_blocks(waiter);
__locks_delete_block(waiter);
+
+ /*
+ * The setting of fl_blocker to NULL marks the "done" point in deleting
+ * a block. Paired with acquire at the top of this function.
+ */
+ smp_store_release(&waiter->fl_blocker, NULL);
spin_unlock(&blocked_lock_lock);
return status;
}
@@ -1350,7 +1387,8 @@ static int posix_lock_inode_wait(struct inode *inode, struct file_lock *fl)
error = posix_lock_inode(inode, fl, NULL);
if (error != FILE_LOCK_DEFERRED)
break;
- error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+ error = wait_event_interruptible(fl->fl_wait,
+ list_empty(&fl->fl_blocked_member));
if (error)
break;
}
@@ -1435,7 +1473,8 @@ int locks_mandatory_area(struct inode *inode, struct file *filp, loff_t start,
error = posix_lock_inode(inode, &fl, NULL);
if (error != FILE_LOCK_DEFERRED)
break;
- error = wait_event_interruptible(fl.fl_wait, !fl.fl_blocker);
+ error = wait_event_interruptible(fl.fl_wait,
+ list_empty(&fl.fl_blocked_member));
if (!error) {
/*
* If we've been sleeping someone might have
@@ -1638,7 +1677,8 @@ restart:
locks_dispose_list(&dispose);
error = wait_event_interruptible_timeout(new_fl->fl_wait,
- !new_fl->fl_blocker, break_time);
+ list_empty(&new_fl->fl_blocked_member),
+ break_time);
percpu_down_read(&file_rwsem);
spin_lock(&ctx->flc_lock);
@@ -2122,7 +2162,8 @@ static int flock_lock_inode_wait(struct inode *inode, struct file_lock *fl)
error = flock_lock_inode(inode, fl);
if (error != FILE_LOCK_DEFERRED)
break;
- error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+ error = wait_event_interruptible(fl->fl_wait,
+ list_empty(&fl->fl_blocked_member));
if (error)
break;
}
@@ -2399,7 +2440,8 @@ static int do_lock_file_wait(struct file *filp, unsigned int cmd,
error = vfs_lock_file(filp, cmd, fl, NULL);
if (error != FILE_LOCK_DEFERRED)
break;
- error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
+ error = wait_event_interruptible(fl->fl_wait,
+ list_empty(&fl->fl_blocked_member));
if (error)
break;
}