On Mon, Aug 08, 2016 at 12:08:18PM -0400, Theodore Ts'o wrote: > On Sun, Aug 07, 2016 at 11:28:10PM -0700, Darrick J. Wong wrote: > > > > I have one lingering concern -- is it a bug that two processes could be > > computing the checksum of a buffer simultaneously? I would have thought ext4 > > would serialize that kind of buffer_head access... > > Do we know how this is happening? We've always depended on the VFS to > provide this exclusion. The only way we should be modifying the > buffer_head at the same time if two CPU's are trying to modify the > directory at the same time, and that should _never_ be happening, even > with the new directory parallism code, unless the file system has > given permission and intends to do its own fine-grained locking. It's a combination of two things, I think. The first is that the checksum calculation routine (temporarily) set the checksum field to zero during the computation, which of course is a no-no. The patch fixes that problem and should go in. The second problem is that we now can have multiple lookups at the same time, which means that there can be more than one CPU calling into dx_probe on the same directory blocks at the same time. There isn't any locking on the buffer heads between readers, so we can end up with ext4_read_dirblock racing with itself to verify the block. It's perhaps a little inefficient for multiple threads to be checksumming the same block, but only turns deadly if you combine it with the first problem. --D > > - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
