On Fri, May 31, 2019 at 04:28:36PM +0200, Roman Penyaev wrote: > On 2019-05-31 14:53, Peter Zijlstra wrote: > > On Fri, May 31, 2019 at 01:15:21PM +0200, Roman Penyaev wrote: > > > On 2019-05-31 11:56, Peter Zijlstra wrote: > > > > On Thu, May 16, 2019 at 10:58:03AM +0200, Roman Penyaev wrote: > > > > > > > + i = __atomic_fetch_add(&ep->user_header->tail, 1, > > > > > + __ATOMIC_ACQUIRE); > > > > > > > > afaict __atomic_fetch_add() does not exist. > > > > > > That is gcc extension. I did not find any API just to increment > > > the variable atomically without using/casting to atomic. What > > > is a proper way to achieve that? > > > > That's C11 atomics, and those shall not be used in the kernel. For one > > they're not available in the minimally required GCC version (4.6). > > > > The proper and only way is to use atomic_t, but also you cannot share > > atomic_t with userspace. > > Yes, that what I tried to avoid choosing c11 extension. > > > > > The normal way of doing something like this is to have a kernel private > > atomic_t and copy the value out to userspace using smp_store_release(). > > Since this path is lockless unfortunately that won't work. So seems > the only way is to do one more cmpxchg (sigh) or give up and take a > look (sad sigh). Of course it works; most of the extant buffers already have a shadow tail and do the update to userspace with a store-release.
