Messages in this thread | | | Date | Sun, 28 Jun 2009 13:22:14 +0200 | From | Jarek Poplawski <> | Subject | Re: [PATCH] net: fix race in the receive/select |
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Jarek Poplawski wrote, On 06/28/2009 01:10 PM:
> 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(); > } > } >
Hmm... of course:
static inline void sock_poll_wait(struct file * filp, struct sock *sk, poll_table *p) { if (p && sk->sk_sleep) { __poll_wait(filp, sk->sk_sleep, p); /* * fat comment */ smp_mb(); // or smp_mb__after_unlock(); } }
Jarek P.
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