Messages in this thread | | | Date | Mon, 24 Nov 2014 18:19:00 +0100 | From | Angelo Rizzi <> | Subject | Re: net_tx_action race condition? |
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Hi Eric, Thanks for your reply You are right, it seems a bug in the NIOS2 architecture port. I will check how local_irq_disable()/local_irq_enable() is implemented on this kind of architecture.
Regards, Angelo
Il 24/11/2014 16:33, Eric Dumazet ha scritto: > On Mon, 2014-11-24 at 14:29 +0100, Angelo Rizzi wrote: >> Hi Daniel, >> Here attached the patch file you required. >> The problem i've found is on the declaration of 'struct softnet_data >> *sd' in function 'net_tx_action' >> What happens to me (i have an embedded system based on FPGA and a NIOS2 >> cpu) is that, due to compiler optimization, the content of >> 'sd->completion_queue' is saved in a CPU register before interrupt >> disabling (when the instruction 'if (sd->completion_queue) {' is >> executed) and then the register contents is used for interrupt-disabled >> assignment ('clist = sd->completion_queue') instead of re-read the >> variable contents. >> This seems to lead to a race condition when an interrupt modifies the >> content of 'sd->completion_queue' between these two instructions. >> What i have done to avoid this situation is to change the declaration of >> 'struct softnet_data *sd' to 'volatile struct softnet_data *sd' and now >> everything seems to be ok. >> I hope this will help. >> >> Regards, >> Angelo >> > Do not add volatile in the kernel, this is not how we solve this kind of > problems. Documentation/memory-barriers.txt and > > Documentation/00-INDEX:476:volatile-considered-harmful.txt > Documentation/00-INDEX:477: - Why the "volatile" type class should not be used > > > I am surprised this patch is needed. Many other 'bugs' would need > similar fixes. > > local_irq_disable() MUST have a memory barrier. This looks like a bug in > one particular arch implementation. > > > On x86 for example, native_irq_disable() is really : > > asm volatile("cli": : :"memory"); > > > diff --git a/net/core/dev.c b/net/core/dev.c > index ac4836241a965a71952469ba054f87d8dfca2d32..fa73072e515aa07fa8ae1bc39174b7d59c7a31a5 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -3406,7 +3406,7 @@ static void net_tx_action(struct softirq_action *h) > struct sk_buff *clist; > > local_irq_disable(); > - clist = sd->completion_queue; > + clist = ACCESS_ONCE(sd->completion_queue); > sd->completion_queue = NULL; > local_irq_enable(); > > > >
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