diff options
| -rw-r--r-- | net/atm/pppoatm.c | 95 |
1 files changed, 85 insertions, 10 deletions
diff --git a/net/atm/pppoatm.c b/net/atm/pppoatm.c index 614d3fc47ede..ce1e59fdae7b 100644 --- a/net/atm/pppoatm.c +++ b/net/atm/pppoatm.c @@ -62,12 +62,25 @@ struct pppoatm_vcc { void (*old_pop)(struct atm_vcc *, struct sk_buff *); /* keep old push/pop for detaching */ enum pppoatm_encaps encaps; + atomic_t inflight; + unsigned long blocked; int flags; /* SC_COMP_PROT - compress protocol */ struct ppp_channel chan; /* interface to generic ppp layer */ struct tasklet_struct wakeup_tasklet; }; /* + * We want to allow two packets in the queue. The one that's currently in + * flight, and *one* queued up ready for the ATM device to send immediately + * from its TX done IRQ. We want to be able to use atomic_inc_not_zero(), so + * inflight == -2 represents an empty queue, -1 one packet, and zero means + * there are two packets in the queue. + */ +#define NONE_INFLIGHT -2 + +#define BLOCKED 0 + +/* * Header used for LLC Encapsulated PPP (4 bytes) followed by the LCP protocol * ID (0xC021) used in autodetection */ @@ -102,16 +115,30 @@ static void pppoatm_wakeup_sender(unsigned long arg) static void pppoatm_pop(struct atm_vcc *atmvcc, struct sk_buff *skb) { struct pppoatm_vcc *pvcc = atmvcc_to_pvcc(atmvcc); + pvcc->old_pop(atmvcc, skb); + atomic_dec(&pvcc->inflight); + /* - * We don't really always want to do this since it's - * really inefficient - it would be much better if we could - * test if we had actually throttled the generic layer. - * Unfortunately then there would be a nasty SMP race where - * we could clear that flag just as we refuse another packet. - * For now we do the safe thing. + * We always used to run the wakeup tasklet unconditionally here, for + * fear of race conditions where we clear the BLOCKED flag just as we + * refuse another packet in pppoatm_send(). This was quite inefficient. + * + * In fact it's OK. The PPP core will only ever call pppoatm_send() + * while holding the channel->downl lock. And ppp_output_wakeup() as + * called by the tasklet will *also* grab that lock. So even if another + * CPU is in pppoatm_send() right now, the tasklet isn't going to race + * with it. The wakeup *will* happen after the other CPU is safely out + * of pppoatm_send() again. + * + * So if the CPU in pppoatm_send() has already set the BLOCKED bit and + * it about to return, that's fine. We trigger a wakeup which will + * happen later. And if the CPU in pppoatm_send() *hasn't* set the + * BLOCKED bit yet, that's fine too because of the double check in + * pppoatm_may_send() which is commented there. */ - tasklet_schedule(&pvcc->wakeup_tasklet); + if (test_and_clear_bit(BLOCKED, &pvcc->blocked)) + tasklet_schedule(&pvcc->wakeup_tasklet); } /* @@ -184,6 +211,51 @@ error: ppp_input_error(&pvcc->chan, 0); } +static inline int pppoatm_may_send(struct pppoatm_vcc *pvcc, int size) +{ + /* + * It's not clear that we need to bother with using atm_may_send() + * to check we don't exceed sk->sk_sndbuf. If userspace sets a + * value of sk_sndbuf which is lower than the MTU, we're going to + * block for ever. But the code always did that before we introduced + * the packet count limit, so... + */ + if (atm_may_send(pvcc->atmvcc, size) && + atomic_inc_not_zero_hint(&pvcc->inflight, NONE_INFLIGHT)) + return 1; + + /* + * We use test_and_set_bit() rather than set_bit() here because + * we need to ensure there's a memory barrier after it. The bit + * *must* be set before we do the atomic_inc() on pvcc->inflight. + * There's no smp_mb__after_set_bit(), so it's this or abuse + * smp_mb__after_clear_bit(). + */ + test_and_set_bit(BLOCKED, &pvcc->blocked); + + /* + * We may have raced with pppoatm_pop(). If it ran for the + * last packet in the queue, *just* before we set the BLOCKED + * bit, then it might never run again and the channel could + * remain permanently blocked. Cope with that race by checking + * *again*. If it did run in that window, we'll have space on + * the queue now and can return success. It's harmless to leave + * the BLOCKED flag set, since it's only used as a trigger to + * run the wakeup tasklet. Another wakeup will never hurt. + * If pppoatm_pop() is running but hasn't got as far as making + * space on the queue yet, then it hasn't checked the BLOCKED + * flag yet either, so we're safe in that case too. It'll issue + * an "immediate" wakeup... where "immediate" actually involves + * taking the PPP channel's ->downl lock, which is held by the + * code path that calls pppoatm_send(), and is thus going to + * wait for us to finish. + */ + if (atm_may_send(pvcc->atmvcc, size) && + atomic_inc_not_zero(&pvcc->inflight)) + return 1; + + return 0; +} /* * Called by the ppp_generic.c to send a packet - returns true if packet * was accepted. If we return false, then it's our job to call @@ -207,7 +279,7 @@ static int pppoatm_send(struct ppp_channel *chan, struct sk_buff *skb) struct sk_buff *n; n = skb_realloc_headroom(skb, LLC_LEN); if (n != NULL && - !atm_may_send(pvcc->atmvcc, n->truesize)) { + !pppoatm_may_send(pvcc, n->truesize)) { kfree_skb(n); goto nospace; } @@ -215,12 +287,12 @@ static int pppoatm_send(struct ppp_channel *chan, struct sk_buff *skb) skb = n; if (skb == NULL) return DROP_PACKET; - } else if (!atm_may_send(pvcc->atmvcc, skb->truesize)) + } else if (!pppoatm_may_send(pvcc, skb->truesize)) goto nospace; memcpy(skb_push(skb, LLC_LEN), pppllc, LLC_LEN); break; case e_vc: - if (!atm_may_send(pvcc->atmvcc, skb->truesize)) + if (!pppoatm_may_send(pvcc, skb->truesize)) goto nospace; break; case e_autodetect: @@ -285,6 +357,9 @@ static int pppoatm_assign_vcc(struct atm_vcc *atmvcc, void __user *arg) if (pvcc == NULL) return -ENOMEM; pvcc->atmvcc = atmvcc; + + /* Maximum is zero, so that we can use atomic_inc_not_zero() */ + atomic_set(&pvcc->inflight, NONE_INFLIGHT); pvcc->old_push = atmvcc->push; pvcc->old_pop = atmvcc->pop; pvcc->encaps = (enum pppoatm_encaps) be.encaps; |