Fix "To" and "Cc".. Li Zefan wrote: > Ben Blum wrote: >> On Tue, Dec 08, 2009 at 03:38:43PM +0800, Li Zefan wrote: >>>> @@ -1291,6 +1324,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type, >>>> struct cgroupfs_root *new_root; >>>> >>>> /* First find the desired set of subsystems */ >>>> + down_read(&subsys_mutex); >>> Hmm.. this can lead to deadlock. sget() returns success with sb->s_umount >>> held, so here we have: >>> >>> down_read(&subsys_mutex); >>> >>> down_write(&sb->s_umount); >>> >>> On the other hand, sb->s_umount is held before calling kill_sb(), >>> so when umounting we have: >>> >>> down_write(&sb->s_umount); >>> >>> down_read(&subsys_mutex); >> Unless I'm gravely mistaken, you can't have deadlock on an rwsem when >> it's being taken for reading in both cases? You would have to have at >> least one of the cases being down_write. >> > > lockdep will warn on this.. > > And it can really lead to deadlock, though not so obivously: > > thread 1 thread 2 thread 3 > ------------------------------------------- > | read(A) write(B) > | > | write(A) > | > | read(A) > | > | write(B) > | > > t3 is waiting for t1 to release the lock, then t2 tries to > acquire A lock to read, but it has to wait because of t3, > and t1 has to wait t2. > > Note: a read lock has to wait if a write lock is already > waiting for the lock. > >> In fairness to readability, perhaps subsys_mutex should instead be >> subsys_rwsem? It seemed to me to be that calling it "mutex" was >> conventional anyway. >> _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
