On Mon, Sep 29, 2014 at 04:00:09PM +0100, Matt Fleming wrote: > On Mon, 29 Sep, at 04:05:16PM, Peter Zijlstra wrote: > > > > OMFG what a trainwreck... if they are reentrant like that, a lock isn't > > going to help you in any way. The implementation of these calls must be > > lockfree otherwise they cannot possibly be correct. > > The only way these services are going to be called is if we (the OS) > invoke them. These NMI shenanigans we're playing in the locking > functions are actually for our benefit, not the firmware's. > > > Conditional locking like above is just plain broken, disgusting doesn't > > even begin to cover it. Full NAK on this. If this is required by the EFI > > spec someone needs to pull their head from their arse and smell the real > > world. > > The conditional locking isn't intended to conform to the spec, it's > intended to write a pstore object to the EFI variable NVRAM with our > last dying breath, even if someone was in the middle of a SetVariable() > call. All the spec says is that, if we're in a non-recoverable state, > it's OK to do that. Whether that'll be implemented correctly across a > range of firmware implementations is another matter. > > In fact, maybe we shouldn't support that lockless access at all. I've no > huge problem changing the code in efi_pstore_write() to bail if we can't > grab the lock, so that we can be serialized all of the time. > > That would certainly make the runtime lock code much simpler. Right, like we talked about on IRC, we need to either drop all from NMI stuff or do the trylock-from-NMI thing you suggested and have a runtime test to make sure that all actually works. Because just not having any serialization is relying on the firmware to not screw itself, which I think is unsound, esp. given that Windows is unlikely to rely on this and we all know the quality of implementation, esp. outside of what Windows does. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
