Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

NeilBrown
2020-03-12T22:19:37Z

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On Thu, Mar 12 2020, Jeff Layton wrote:

> On Thu, 2020-03-12 at 15:42 +1100, NeilBrown wrote:
>> On Wed, Mar 11 2020, Linus Torvalds wrote:
>>
>> > On Wed, Mar 11, 2020 at 3:22 PM NeilBrown <neilb@suse.de> wrote:
>> > > We can combine the two ideas - move the list_del_init() later, and still
>> > > protect it with the wq locks. This avoids holding the lock across the
>> > > callback, but provides clear atomicity guarantees.
>> >
>> > Ugfh. Honestly, this is disgusting.
>> >
>> > Now you re-take the same lock in immediate succession for the
>> > non-callback case. It's just hidden.
>> >
>> > And it's not like the list_del_init() _needs_ the lock (it's not
>> > currently called with the lock held).
>> >
>> > So that "hold the lock over list_del_init()" seems to be horrendously
>> > bogus. It's only done as a serialization thing for that optimistic
>> > case.
>> >
>> > And that optimistic case doesn't even *want* that kind of
>> > serialization. It really just wants a "I'm done" flag.
>> >
>> > So no. Don't do this. It's mis-using the lock in several ways.
>> >
>> > Linus
>>
>> It seems that test_and_set_bit_lock() is the preferred way to handle
>> flags when memory ordering is important, and I can't see how to use that
>> well with an "I'm done" flag. I can make it look OK with a "I'm
>> detaching" flag. Maybe this is better.
>>
>> NeilBrown
>>
>> From f46db25f328ddf37ca9fbd390c6eb5f50c4bd2e6 Mon Sep 17 00:00:00 2001
>> From: NeilBrown <neilb@suse.de>
>> Date: Wed, 11 Mar 2020 07:39:04 +1100
>> Subject: [PATCH] locks: restore locks_delete_lock optimization
>>
>> A recent patch (see Fixes: below) removed an optimization which is
>> important as it avoids taking a lock in a common case.
>>
>> The comment justifying the optimisation was correct as far as it went,
>> in that if the tests succeeded, then the values would remain stable and
>> the test result will remain valid even without a lock.
>>
>> However after the test succeeds the lock can be freed while some other
>> thread might have only just set ->blocker to NULL (thus allowing the
>> test to succeed) but has not yet called wake_up() on the wq in the lock.
>> If the wake_up happens after the lock is freed, a use-after-free error occurs.
>>
>> This patch restores the optimization and adds a flag to ensure this
>> use-after-free is avoid. The use happens only when the flag is set, and
>> the free doesn't happen until the flag has been cleared, or we have
>> taken blocked_lock_lock.
>>
>> Fixes: 6d390e4b5d48 ("locks: fix a potential use-after-free problem when wakeup a waiter")
>> Signed-off-by: NeilBrown <neilb@suse.de>
>> ---
>> fs/locks.c | 44 ++++++++++++++++++++++++++++++++++++++------
>> include/linux/fs.h | 3 ++-
>> 2 files changed, 40 insertions(+), 7 deletions(-)
>>
>
> Just a note that I'm traveling at the moment, and won't be able do much
> other than comment on this for a few days.
>
>> diff --git a/fs/locks.c b/fs/locks.c
>> index 426b55d333d5..334473004c6c 100644
>> --- a/fs/locks.c
>> +++ b/fs/locks.c
>> @@ -283,7 +283,7 @@ locks_dump_ctx_list(struct list_head *list, char *list_type)
>> struct file_lock *fl;
>>
>> list_for_each_entry(fl, list, fl_list) {
>> - pr_warn("%s: fl_owner=%p fl_flags=0x%x fl_type=0x%x fl_pid=%u\n", list_type, fl->fl_owner, fl->fl_flags, fl->fl_type, fl->fl_pid);
>> + pr_warn("%s: fl_owner=%p fl_flags=0x%lx fl_type=0x%x fl_pid=%u\n", list_type, fl->fl_owner, fl->fl_flags, fl->fl_type, fl->fl_pid);
>> }
>> }
>>
>> @@ -314,7 +314,7 @@ locks_check_ctx_file_list(struct file *filp, struct list_head *list,
>> list_for_each_entry(fl, list, fl_list)
>> if (fl->fl_file == filp)
>> pr_warn("Leaked %s lock on dev=0x%x:0x%x ino=0x%lx "
>> - " fl_owner=%p fl_flags=0x%x fl_type=0x%x fl_pid=%u\n",
>> + " fl_owner=%p fl_flags=0x%lx fl_type=0x%x fl_pid=%u\n",
>> list_type, MAJOR(inode->i_sb->s_dev),
>> MINOR(inode->i_sb->s_dev), inode->i_ino,
>> fl->fl_owner, fl->fl_flags, fl->fl_type, fl->fl_pid);
>> @@ -736,10 +736,13 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
>> waiter = list_first_entry(&blocker->fl_blocked_requests,
>> struct file_lock, fl_blocked_member);
>> __locks_delete_block(waiter);
>> - if (waiter->fl_lmops && waiter->fl_lmops->lm_notify)
>> - waiter->fl_lmops->lm_notify(waiter);
>> - else
>> - wake_up(&waiter->fl_wait);
>> + if (!test_and_set_bit_lock(FL_DELETING, &waiter->fl_flags)) {
>> + if (waiter->fl_lmops && waiter->fl_lmops->lm_notify)
>> + waiter->fl_lmops->lm_notify(waiter);
>> + else
>> + wake_up(&waiter->fl_wait);
>> + clear_bit_unlock(FL_DELETING, &waiter->fl_flags);
>> + }
>
> I *think* this is probably safe.
>
> AIUI, when you use atomic bitops on a flag word like this, you should
> use them for all modifications to ensure that your changes don't get
> clobbered by another task racing in to do a read/modify/write cycle on
> the same word.
>
> I haven't gone over all of the places where fl_flags is changed, but I
> don't see any at first glance that do it on a blocked request.
>
>> }
>> }
>>
>> @@ -753,11 +756,40 @@ 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. So it is safe to test fl_blocker locklessly.
>> + * Also if fl_blocker is NULL, this waiter is not listed on
>> + * fl_blocked_requests for some lock, so no other request can
>> + * be added to the list of fl_blocked_requests for this
>> + * request. So if fl_blocker is NULL, it is safe to
>> + * locklessly check if fl_blocked_requests is empty. If both
>> + * of these checks succeed, there is no need to take the lock.
>> + *
>> + * We perform these checks only if we can set FL_DELETING.
>> + * This ensure that we don't race with __locks_wake_up_blocks()
>> + * in a way which leads it to call wake_up() *after* we return
>> + * and the file_lock is freed.
>> + */
>> + if (!test_and_set_bit_lock(FL_DELETING, &waiter->fl_flags)) {
>> + if (waiter->fl_blocker == NULL &&
>> + list_empty(&waiter->fl_blocked_requests)) {
>> + /* Already fully unlinked */
>> + clear_bit_unlock(FL_DELETING, &waiter->fl_flags);
>> + return status;
>> + }
>> + }
>> +
>> spin_lock(&blocked_lock_lock);
>> if (waiter->fl_blocker)
>> status = 0;
>> __locks_wake_up_blocks(waiter);
>> __locks_delete_block(waiter);
>> + /* This flag might not be set and it is largely irrelevant
>> + * now, but it seem cleaner to clear it.
>> + */
>> + clear_bit(FL_DELETING, &waiter->fl_flags);
>> spin_unlock(&blocked_lock_lock);
>> return status;
>> }
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 3cd4fe6b845e..4db514f29bca 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -1012,6 +1012,7 @@ static inline struct file *get_file(struct file *f)
>> #define FL_UNLOCK_PENDING 512 /* Lease is being broken */
>> #define FL_OFDLCK 1024 /* lock is "owned" by struct file */
>> #define FL_LAYOUT 2048 /* outstanding pNFS layout */
>> +#define FL_DELETING 32768 /* lock is being disconnected */
>
> nit: Why the big gap?

No good reason - it seems like a conceptually different sort of flag so
I vaguely felt that it would help if it were numerically separate.

>
>>
>> #define FL_CLOSE_POSIX (FL_POSIX | FL_CLOSE)
>>
>> @@ -1087,7 +1088,7 @@ struct file_lock {
>> * ->fl_blocker->fl_blocked_requests
>> */
>> fl_owner_t fl_owner;
>> - unsigned int fl_flags;
>> + unsigned long fl_flags;
>
> This will break kABI, so backporting this to enterprise distro kernels
> won't be trivial. Not a showstopper, but it might be nice to avoid that
> if we can.
>
> While it's not quite as efficient, we could just do the FL_DELETING
> manipulation under the flc->flc_lock. That's per-inode, so it should be
> safe to do it that way.

If we are going to use a spinlock, I'd much rather not add a flag bit,
but instead use the blocked_member list_head.

I'm almost tempted to suggest adding
smp_list_del_init_release() and smp_list_empty_careful_acquire()
so that list membership can be used as a barrier. I'm not sure I game
though.

NeilBrown

>
>> unsigned char fl_type;
>> unsigned int fl_pid;
>> int fl_link_cpu; /* what cpu's list is this on? */

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