From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexey.Brodkin@synopsys.com (Alexey Brodkin) Date: Wed, 14 Mar 2018 20:38:53 +0000 Subject: arc_usr_cmpxchg and preemption In-Reply-To: <20180314175352.GP4064@hirez.programming.kicks-ass.net> References: <1521045375.11552.27.camel@synopsys.com> <20180314175352.GP4064@hirez.programming.kicks-ass.net> List-ID: Message-ID: <1521059931.11552.51.camel@synopsys.com> To: linux-snps-arc@lists.infradead.org Hi Peter, Vineet, On Wed, 2018-03-14@18:53 +0100, Peter Zijlstra wrote: > On Wed, Mar 14, 2018@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; I guess this jump we need to do only once, right? If for whatever reason get_user_pages_fast() fails we return immediately and if it succeeds there's no reason for put_user() to not succeed as required page is supposed to be prepared for write. Otherwise if something goes way too bad we may end-up in an infinite loop which we'd better prevent. > } > > return ret; > } @Vineet, are you OK with proposed implementation? -Alexey