On Mon, 2 Nov 2020 17:41:47 +0100 Petr Mladek <pmladek@xxxxxxxx> wrote: > > + i = atomic_read(&nr_records); > > + smp_mb__after_atomic(); > > + if (i < 0) > > + cmpxchg(&recursed_functions[index].ip, ip, 0); > > + else if (i <= index) > > + atomic_cmpxchg(&nr_records, i, index + 1); > > This looks weird. It would shift nr_records past the record added > in this call. It might skip many slots that were zeroed when clearing. > Also we do not know if our entry was not zeroed as well. nr_records always holds the next position to write to. index = nr_records; recursed_functions[index].ip = ip; nr_records++; Before clearing, we have: nr_records = -1; smp_mb(); memset(recursed_functions, 0); smp_wmb(); nr_records = 0; When we enter this function: i = nr_records; smp_mb(); if (i < 0) return; Thus, we just stopped all new updates while clearing the records. But what about if something is currently updating? i = nr_records; smp_mb(); if (i < 0) cmpxchg(recursed_functions, ip, 0); The above shows that if the current updating process notices that the clearing happens, it will clear the function it added. else if (i <= index) cmpxchg(nr_records, i, index + 1); This makes sure that nr_records only grows if it is greater or equal to zero. The only race that I see that can happen, is the one in the comment I showed. And that is after enabling the recursed functions again after clearing, one CPU could add a function while another CPU that just added that same function could be just exiting this routine, notice that a clearing of the array happened, and remove its function (which was the same as the one just happened). So we get a "zero" in the array. If this happens, it is likely that that function will recurse again and will be added later. -- Steve
