Messages in this thread | | | Date | Sun, 28 Jun 2009 13:10:45 +0200 | From | Jarek Poplawski <> | Subject | Re: [PATCH] net: fix race in the receive/select |
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Oleg Nesterov wrote, On 06/26/2009 04:50 PM:
> On 06/26, Davide Libenzi wrote: >> On Fri, 26 Jun 2009, Oleg Nesterov wrote: >> >>> And if we remove waitqueue_active() in xxx_update(), then lock/unlock is >>> not needed too. >>> >>> If xxx_poll() takes q->lock first, it can safely miss the changes in ->status >>> and schedule(): xxx_update() will take q->lock, notice the sleeper and wake >>> it up (ok, it will set ->triggered but this doesn't matter). >>> >>> If xxx_update() takes q->lock first, xxx_poll() must see the changes in >>> status after poll_wait()->unlock(&q->lock) (in fact, after lock, not unlock). >> Sure. The snippet above was just to show what typically the code does, not >> a suggestion on how to solve the socket case. > > Yes, yes. I just meant you are right imho, we shouldn't add mb() into > add_wait_queue(). > >> But yeah, the problem in this case is the waitqueue_active() call. Without >> that, the wait queue lock/unlock in poll_wait() and the one in wake_up() >> guarantees the necessary barriers. >> Some might argue the costs of the lock/unlock of q->lock, and wonder if >> MBs are a more efficient solution. This is something I'm not going into. >> To me, it just looked not right having cross-matching MB in different >> subsystems. > > This is subjective and thus up to maintainers, but personally I think you > are very, very right. > > Perhaps we can add > > void sock_poll_wait(struct file *file, struct sock *sk, poll_table *pt) > { > if (pt) { > poll_wait(file, sk->sk_sleep, pt); > /* > * fat comment > */ > smp_mb(); // or smp_mb__after_unlock(); > } > } > > Oleg.
Maybe 'a bit' further?:
static inline void __poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p) { p->qproc(filp, wait_address, p); }
static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p) { if (p && wait_address) __poll_wait(filp, wait_address, p); }
static inline void sock_poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p) { if (p && wait_address) { __poll_wait(filp, wait_address, p); /* * fat comment */ smp_mb(); // or smp_mb__after_unlock(); } }
Jarek P.
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