Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes: > On Fri, Nov 08, 2013 at 12:51:52PM -0800, Eric W. Biederman wrote: > >> The return value of d_mountpoint can be obsolete as soon as it returns >> as well, so I don't see this as being significantly different. > > Not if the ->i_mutex of that sucker is held. And it *is* held in > vfs_unlink/vfs_rmdir/vfs_rename. Note that we only care about a mountpoint > being falsely assumed to be a non-mountpoint - in the other direction we > can just shrug and say that we'd won the race and got EBUSY for that. I wasn't certain of your question. My point here was that covered() as a mechanism is as good as d_mountpoint. So the only potential issue with covered() as a mechanism is where covered() is called. Also please note old_dentry->d_inode->i_mutex is not held in rename. >> In 3.12 vfs_rmdir checks d_mountpoint with the >> dentry->d_inode->i_mutex and >> dentry->d_parent->d_inode->i_mutex held. >> >> In 3.12 vfs_unlink checks d_mountpoint with the >> dentry->d_inode->i_mutex and >> dentry->d_parent->d_inode->i_mutex hel.d >> >> In 3.12 vfs_rename_dir and vfs_rename_other checks d_mountpint with the >> target->i_mutex, new_dir->i_mutex, and old_dir->i_mutex held. >> >> >> Therefore the guarantees in 3.12 are: >> - unlink versus mount races are prevented by the >> dentry->d_inode->i_mutex of the dentry being removed. >> - unlink versus umount races are uninteresting. >> - mount versus rename races in testing of d_mountpoint are ignored. > > Read what you've written a few lines above. The part about target->i_mutex > being held. That works for the rename as unlink case but we don't hold old_dentry->d_inode->i_mutex which is what is needed to prevent a mount on the dentry we are renaming. >> So comparing this to how I have implemented covered the test is at a >> slightly different location in the call path so there may be a slightly >> larger race in rename. > > You've got a race in unlink. You've got a race in rename. You've got a race > in rmdir. And none of those had that race in 3.12 (including rename()). Rename absolutely has a race in 3.12. With very lucky timing it is possible to mount something on directory a, simultaneously rename a to b, and have the mount show up on b. > BTW, could you describe the races with umount in a bit more details? Races > with mount are simple - rmdir() sees that victim isn't a mountpoint and > proceeds, mount() sees that victim is still alive and proceeds, despite > the fact that victim is irretrievably on the way to removal. And that's > what ->i_mutex on victim prevents, making "check for d_mountpoint / remove / > call dont_mount()" atomic wrt mount(). What is the problem you are seeing > with umount()? rmdir() getting EBUSY because it hasn't noticed umount() > happening in parallel with it? Legitimate behaviour, as far I can see... > Or is it about something different? I did not say it was a problem only that it was a race. The only case I can see is getting a state EBUSY, and I see no problem with a that. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
