Hi Mathieu, Sorry for the delay in responding.
On Fri, Sep 13, 2019 at 10:22:28AM -0400, Mathieu Desnoyers wrote: > ----- On Sep 12, 2019, at 11:47 AM, Will Deacon w...@kernel.org wrote: > > > On Thu, Sep 12, 2019 at 03:24:35PM +0100, Linus Torvalds wrote: > >> On Thu, Sep 12, 2019 at 2:48 PM Will Deacon <w...@kernel.org> wrote: > >> > > >> > So the man page for sys_membarrier states that the expedited variants > >> > "never > >> > block", which feels pretty strong. Do any other system calls claim to > >> > provide this guarantee without a failure path if blocking is necessary? > >> > >> The traditional semantics for "we don't block" is that "we block on > >> memory allocations and locking and user accesses etc, but we don't > >> wait for our own IO". > >> > >> So there may be new IO started (and waited on) as part of allocating > >> new memory etc, or in just paging in user memory, but the IO that the > >> operation _itself_ explicitly starts is not waited on. > > > > Thanks, that makes sense, and I'd be inclined to suggest an update to the > > sys_membarrier manpage to make this more clear since the "never blocks" > > phrasing doesn't seem to be used like this for other system calls. > > The current wording from membarrier(2) is: > > The "expedited" commands complete faster than the non-expedited > ones; they never block, but have the downside of causing extra > overhead. > > We could simply remove the "; they never block" part then ? I think so, yes. That or, "; they do not voluntarily block" or something like that. Maybe look at other man pages for inspiration ;) > >> No system call should ever be considered "atomic" in any sense. If > >> you're doing RT, you should maybe expect "getpid()" to not ever block, > >> but that's just about the exclusive list of truly nonblocking system > >> calls, and even that can be preempted. > > > > In which case, why can't we just use GFP_KERNEL for the cpumask allocation > > instead of GFP_NOWAIT and then remove the failure path altogether? Mathieu? > > Looking at: > > #define GFP_KERNEL (__GFP_RECLAIM | __GFP_IO | __GFP_FS) > > I notice that it does not include __GFP_NOFAIL. What prevents GFP_KERNEL from > failing, and where is this guarantee documented ? There was an lwn article a little while ago about this: https://lwn.net/Articles/723317/ I'm not sure what (if anything) has changed in this regard since then, however. > Regarding __GFP_NOFAIL, its use seems to be discouraged in linux/gfp.h: > > * %__GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller > * cannot handle allocation failures. The allocation could block > * indefinitely but will never return with failure. Testing for > * failure is pointless. > * New users should be evaluated carefully (and the flag should be > * used only when there is no reasonable failure policy) but it is > * definitely preferable to use the flag rather than opencode endless > * loop around allocator. > * Using this flag for costly allocations is _highly_ discouraged. > > So I am reluctant to use it. > > But if we can agree on the right combination of flags that guarantees there > is no failure, I would be perfectly fine with using them to remove the > fallback > code. I reckon you'll be fine using GFP_KERNEL and returning -ENOMEM on allocation failure. This shouldn't happen in practice and it removes the fallback path. Will
