lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Fri, 18 Nov 2016 10:30:47 +0000 From: Sudeep Holla <sudeep.holla@....com> To: Vikram Mulukutla <markivx@...eaurora.org> Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Sudeep Holla <sudeep.holla@....com>, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org Subject: Re: spin_lock behavior with ARM64 big.Little/HMP Hi Vikram, On 18/11/16 02:22, Vikram Mulukutla wrote: > Hello, > > This isn't really a bug report, but just a description of a frequency/IPC > dependent behavior that I'm curious if we should worry about. The behavior > is exposed by questionable design so I'm leaning towards don't-care. > > Consider these threads running in parallel on two ARM64 CPUs running > mainline > Linux: > Are you seeing this behavior with the mainline kernel on any platforms as we have a sort of workaround for this ? > (Ordering of lines between the two columns does not indicate a sequence of > execution. Assume flag=0 initially.) > > LittleARM64_CPU @ 300MHz (e.g.A53) | BigARM64_CPU @ 1.5GHz (e.g. A57) > -------------------------------------+---------------------------------- > spin_lock_irqsave(s) | local_irq_save() > /* critical section */ > flag = 1 | spin_lock(s) > spin_unlock_irqrestore(s) | while (!flag) { > | spin_unlock(s) > | cpu_relax(); > | spin_lock(s) > | } > | spin_unlock(s) > | local_irq_restore() > > I see a livelock occurring where the LittleCPU is never able to acquire the > lock, and the BigCPU is stuck forever waiting on 'flag' to be set. > Yes we saw this issue 3 years back on TC2 which has A7(with lowest frequency of 300MHz IIRC) and A15(with 1.2 GHz). We were observing that inter-cluster events are missed since the two clusters are operating at different frequencies (details below). The hardware recommendation is that there should be glue logic between the two clusters which captures events from one cluster and replays then on the other if its operating at a different frequency. Generally EVENTO from cluster 1 is connected to the EVENTI of the cluster 2 and vice versa. The only extra logic required is the double synchronizer in the receiving clock domain. This issue arise in reality if the synchronizer is missing and different CPUs hold EVENTO for different clock cycles. However there was a different requirement to implement timer event stream in Linux for some user-space locking and that indirectly help to resolve the issue on TC2. That event stream feature is enabled by default in Linux and should fix the issue and hence I asked you if you still see that issue. -- Regards, Sudeep
Powered by blists - more mailing lists