On Mon, Jun 03, 2013 at 11:45:34AM +0100, Russell King - ARM Linux wrote: > On Mon, Jun 03, 2013 at 11:27:23AM +0100, Will Deacon wrote: > > On Mon, Jun 03, 2013 at 11:18:09AM +0100, Russell King - ARM Linux wrote: > > > On Thu, May 30, 2013 at 12:41:12PM +0100, Will Deacon wrote: > > > > +#if defined(CONFIG_OABI_COMPAT) || !defined(CONFIG_AEABI) > > > > + /* > > > > + * We may have faulted trying to load the SWI instruction due to > > > > + * concurrent page aging on another CPU. In this case, return > > > > + * back to the swi instruction and fault the page back. > > > > + */ > > > > +9001: > > > > + sub lr, lr, #4 > > > > + str lr, [sp, #S_PC] > > > > + b ret_fast_syscall > > > > +#endif > > > > > > The comment is wrong. If we get here, it means that the fault from > > > trying to loading the instruction can't be fixed up. Arguably, that > > > should result in a SIGSEGV being sent immediately, but we'll get to > > > that when we then try to re-load the instruction. > > > > Why would we kill the application in this case? The reported problem is > > where one CPU ages the page containing the swi instruction (mkold => > > clears L_PTE_YOUNG => write 0 to the pte) in between the other CPU executing > > the swi and the kernel trying to read the immediate. The VMA is fine. > > If you mark the instruction was a user-accessing instruction, the kernel > will handle the resulting exception, trying to make the page accessible. > If it is successful, then execution resumes as normal at the faulting > instruction and continues as if nothing happened. > > If it can't make the page accessible (eg, out of memory) the exception > handler path (your code above) will be called instead. Normal action in > that case would be for a system call to return -EFAULT, but in this case > we can't know what the syscall was, so we don't know if userspace will > even pay attention to the returned error code. In any case, if the page > is no longer accessible, it's going to end up being killed by a SEGV > when we eventually return to userspace anyway.
Yes, of course, the fault handling will sort out non-fatal faults for us, so I'll update the comment. Thanks, Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
