On Tue, Aug 28, 2012 at 06:06:10PM -0400, Vivek Goyal wrote: > On Tue, Aug 28, 2012 at 10:37:36AM -0700, Kent Overstreet wrote: > > Previously, if we ever try to allocate more than once from the same bio > > set while running under generic_make_request() (i.e. a stacking block > > driver), we risk deadlock. > > > > This is because of the code in generic_make_request() that converts > > recursion to iteration; any bios we submit won't actually be submitted > > (so they can complete and eventually be freed) until after we return - > > this means if we allocate a second bio, we're blocking the first one > > from ever being freed. > > > > Thus if enough threads call into a stacking block driver at the same > > time with bios that need multiple splits, and the bio_set's reserve gets > > used up, we deadlock. > > Hi Kent, > > So above deadlock possibility arises only if multiple threads are doing > multiple splits from same pool and all the threads get blocked on mempool > and don't return from ->make_request function. > > Is there any existing driver in the tree which can cause this deadlock or > it becomes a possibility only when splitting and bcache code shows up? It is emphatically possible with dm, though you would probably need a pathalogical configuration of your targets for it to be possible in practice - at least with the targets currently commonly in use. With most of the newer dm targets coming down the pipe I expect it should be possible under real world configurations, with the caveat that under that kind of memory pressure in practice many other things will fall over first. -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
