On Fri, 2018-11-02 at 16:15 -0600, Jason Gunthorpe wrote: > On Fri, Nov 02, 2018 at 10:07:02PM +0000, Saeed Mahameed wrote: > > On Fri, 2018-11-02 at 14:39 -0700, Eric Dumazet wrote: > > > > > > On 11/02/2018 02:05 PM, Saeed Mahameed wrote: > > > > > > > temp will be mem copied to priv->stats.sw at the end, > > > > memcpy(&priv->stats.sw, &s, sizeof(s)); > > > > > > > > one other way to solve this as suggested by Andrew, is to get > > > > rid > > > > of > > > > the temp var and make it point directly to priv->stats.sw > > > > > > > > > > What about concurrency ? > > > > > > This temp variable is there to make sure concurrent readers of > > > stats > > > might > > > not see mangle data (because another 'reader' just did a memset() > > > and > > > is doing the folding) > > > > > > > > > mlx5e_get_stats() can definitely be run at the same time by > > > multiple > > > threads. > > > > > > > hmm, you are right, i was thinking that mlx5e_get_Stats will > > trigger a > > work to update stats and grab the state_lock, but for sw stats this > > is > > not the case it is done in place. > > That was my guess when I saw this.. the confusing bit is why is there > s and temp, why not just s? > silly cut and paste from mlx4 ! > > BTW memcpy itself is not thread safe. > > At least on 64 bit memcpy will do > 8 byte stores when copying so on > most architectures it will cause individual new or old u64 to be > returned and not a mess.. > > 32 bit will always make a mess. > > If the stats don't update that often then kmalloc'ing a new buffer > and > RCU'ing it into view might be a reasonable alternative to this? > kmalloc is not an option, stats update too often, priv->stats.sw is already a temp, it is needed only to accumulate channels stats into it and then report them to ethtool buffer or ndo_get_stats , both will copy the priv->stats.sw to their own buffers, so there can only be two use cases (two threads), maybe it is best if we maintain two priv- >stats.sw one for each case and use it as a temp for each thread. > Jason
