Messages in this thread | | | Date | Wed, 14 Mar 2018 18:53:52 +0100 | From | Peter Zijlstra <> | Subject | Re: arc_usr_cmpxchg and preemption |
| |
On Wed, Mar 14, 2018 at 09:58:19AM -0700, Vineet Gupta wrote:
> Well it is broken wrt the semantics the syscall is supposed to provide. > Preemption disabling is what prevents a concurrent thread from coming in and > modifying the same location (Imagine a variable which is being cmpxchg > concurrently by 2 threads). > > One approach is to do it the MIPS way, emulate the llsc flag - set it under > preemption disabled section and clear it in switch_to
*shudder*... just catch the -EFAULT, force the write fault and retry.
Something like:
int sys_cmpxchg(u32 __user *user_ptr, u32 old, u32 new) { u32 val; int ret;
again: ret = 0;
preempt_disable(); val = get_user(user_ptr); if (val == old) ret = put_user(new, user_ptr); preempt_enable();
if (ret == -EFAULT) { struct page *page; ret = get_user_pages_fast((unsigned long)user_ptr, 1, 1, &page); if (ret < 0) return ret; put_page(page); goto again; }
return ret; }
| |