Messages in this thread Patch in this message | | | Date | Mon, 20 Oct 2014 11:49:26 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier |
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On Mon, Oct 20, 2014 at 10:11:40AM +0100, Thomas Gleixner wrote: > On Sat, 18 Oct 2014, Davidlohr Bueso wrote: > > On Sat, 2014-10-18 at 13:50 -0700, Linus Torvalds wrote: > > > On Sat, Oct 18, 2014 at 12:58 PM, Davidlohr Bueso <dave@stgolabs.net> wrote: > > > > > > > > And [get/put]_futex_keys() shouldn't even be called for private futexes. > > > > The following patch had some very minor testing on a 60 core box last > > > > night, but passes both Darren's and perf's tests. So I *think* this is > > > > right, but lack of sleep and I overall just don't trust them futexes! > > > > > > Hmm. I don't see the advantage of making the code more complex in > > > order to avoid the functions that are no-ops for the !fshared case? > > > > > > IOW, as far as I can tell, this patch doesn't actually really *change* > > > anything. Am I missing something? > > > > Right, all we do is avoid a NOP, but I don't see how this patch makes > > the code more complex. In fact, the whole idea is to make it easier to > > read and makes the key referencing differences between shared and > > private futexes crystal clear, hoping to mitigate future bugs. > > I tend to disagree. The current code is symetric versus get/drop and > you make it unsymetric by avoiding the drop call with a pointless > extra conditional in all call sites.
Since you mention symmetry, something like below makes the barriers more explicit. However, it removes the implied barrier for get_futex_key_refs and the other calling places need to be verified (requeue_futex and requeue_pi_futex; if barrier is not required here, the result may be slightly more optimal, not sure you would see the difference though).
diff --git a/kernel/futex.c b/kernel/futex.c index f3a3a071283c..5b9d857d0816 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -143,9 +143,7 @@ * * Where (A) orders the waiters increment and the futex value read through * atomic operations (see hb_waiters_inc) and where (B) orders the write - * to futex and the waiters read -- this is done by the barriers in - * get_futex_key_refs(), through either ihold or atomic_inc, depending on the - * futex type. + * to futex and the waiters read (see hb_waiters_pending). * * This yields the following case (where X:=waiters, Y:=futex): * @@ -262,12 +260,6 @@ static struct futex_hash_bucket *futex_queues; static inline void futex_get_mm(union futex_key *key) { atomic_inc(&key->private.mm->mm_count); - /* - * Ensure futex_get_mm() implies a full barrier such that - * get_futex_key() implies a full barrier. This is relied upon - * as full barrier (B), see the ordering comment above. - */ - smp_mb__after_atomic(); } /* @@ -297,6 +289,10 @@ static inline void hb_waiters_dec(struct futex_hash_bucket *hb) static inline int hb_waiters_pending(struct futex_hash_bucket *hb) { + /* + * Full barrier (B), see the ordering comment above. + */ + smp_mb__before_atomic(); #ifdef CONFIG_SMP return atomic_read(&hb->waiters); #else @@ -338,13 +334,11 @@ static void get_futex_key_refs(union futex_key *key) switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) { case FUT_OFF_INODE: - ihold(key->shared.inode); /* implies MB (B) */ + __iget(key->shared.inode); break; case FUT_OFF_MMSHARED: - futex_get_mm(key); /* implies MB (B) */ + futex_get_mm(key); break; - default: - smp_mb(); /* explicit MB (B) */ } } @@ -417,7 +411,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw) if (!fshared) { key->private.mm = mm; key->private.address = address; - get_futex_key_refs(key); /* implies MB (B) */ + get_futex_key_refs(key); return 0; } @@ -524,7 +518,7 @@ again: key->shared.pgoff = basepage_index(page); } - get_futex_key_refs(key); /* implies MB (B) */ + get_futex_key_refs(key); out: unlock_page(page_head);
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