On 11/2/21 08:03, John Hubbard wrote: > On 10/30/21 03:11, Yunfeng Ye wrote: >> After the memory is freed, it may be allocated by other CPUs and has >> been recorded by trace. So the timing sequence of the memory tracing is >> inaccurate. >> >> For example, we expect the following timing sequeuce: >> >> CPU 0 CPU 1 >> >> (1) alloc xxxxxx >> (2) free xxxxxx >> (3) alloc xxxxxx >> (4) free xxxxxx >> >> However, the following timing sequence may occur: >> >> CPU 0 CPU 1 >> >> (1) alloc xxxxxx >> (2) alloc xxxxxx >> (3) free xxxxxx >> (4) free xxxxxx >> >> So place the trace before freeing memory in kmem_cache_free(). > > Hi Yunfeng, > > Like Muchun, I had some difficulty with the above description, but > now I think I get it. :) > > In order to make it easier for others, how about this wording and subject > line, instead: > > > mm, slub: emit the "free" trace report before freeing memory in > kmem_cache_free() > > After the memory is freed, it can be immediately allocated by other > CPUs, before the "free" trace report has been emitted. This causes > inaccurate traces. > > For example, if the following sequence of events occurs: > > CPU 0 CPU 1 > > (1) alloc xxxxxx > (2) free xxxxxx > (3) alloc xxxxxx > (4) free xxxxxx > > ...then they will be inaccurately reported via tracing, so that they > appear to have happened in this order. This makes it look like CPU 1 > somehow managed to allocate mmemory that CPU 0 still had allocated for > itself: > > CPU 0 CPU 1 > > (1) alloc xxxxxx > (2) alloc xxxxxx > (3) free xxxxxx > (4) free xxxxxx > > In order to avoid this, emit the "free xxxxxx" tracing report just > before the actual call to free the memory, instead of just after it. Agree, this wording is better. IIRC the same problem was fixed for mmap_lock trace ordering just recently. >> Signed-off-by: Yunfeng Ye <yeyunfeng@xxxxxxxxxx> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
