On Wed, 2013-05-01 at 03:36 -0400, David Miller wrote: > From: Benjamin Herrenschmidt <b...@kernel.crashing.org> > Date: Wed, 01 May 2013 11:39:53 +1000 > > > I'm not even completely certain bytes are safe to be honest, though > > probably more than bitfields. I'll poke our compiler people. > > Older Alpha only has 32-bit and 64-bit loads and stores, so byte sized > accesses are not atomic, and therefore use racey read-modify-write > sequences.
Right, so what about the following more general fix ? Thanks ! [PATCH v2] af_unix: fix a fatal race with bit fields Using bit fields is dangerous on ppc64/sparc64, as the compiler [1] uses 64bit instructions to manipulate them. If the 64bit word includes any atomic_t or spinlock_t, we can lose critical concurrent changes. This is happening in af_unix, where unix_sk(sk)->gc_candidate/ gc_maybe_cycle/lock share the same 64bit word. This leads to fatal deadlock, as one/several cpus spin forever on a spinlock that will never be available again. A safer way would be to use a long to store flags. This way we are sure compiler/arch wont do bad things. As we own unix_gc_lock spinlock when clearing or setting bits, we can use the non atomic __set_bit()/__clear_bit(). recursion_level can share the same 64bit location with the spinlock, as it is set only with this spinlock held. [1] bug fixed in gcc-4.8.0 : http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52080 Reported-by: Ambrose Feinstein <ambr...@google.com> Signed-off-by: Eric Dumazet <eduma...@google.com> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> Cc: Paul Mackerras <pau...@samba.org> --- include/net/af_unix.h | 5 +++-- net/unix/garbage.c | 12 ++++++------ 2 files changed, 9 insertions(+), 8 deletions(-) diff --git a/include/net/af_unix.h b/include/net/af_unix.h index a8836e8..dbdfd2b 100644 --- a/include/net/af_unix.h +++ b/include/net/af_unix.h @@ -57,9 +57,10 @@ struct unix_sock { struct list_head link; atomic_long_t inflight; spinlock_t lock; - unsigned int gc_candidate : 1; - unsigned int gc_maybe_cycle : 1; unsigned char recursion_level; + unsigned long gc_flags; +#define UNIX_GC_CANDIDATE 0 +#define UNIX_GC_MAYBE_CYCLE 1 struct socket_wq peer_wq; }; #define unix_sk(__sk) ((struct unix_sock *)__sk) diff --git a/net/unix/garbage.c b/net/unix/garbage.c index d0f6545..9c6cc08 100644 --- a/net/unix/garbage.c +++ b/net/unix/garbage.c @@ -185,7 +185,7 @@ static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *), * have been added to the queues after * starting the garbage collection */ - if (u->gc_candidate) { + if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) { hit = true; func(u); } @@ -254,7 +254,7 @@ static void inc_inflight_move_tail(struct unix_sock *u) * of the list, so that it's checked even if it was already * passed over */ - if (u->gc_maybe_cycle) + if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags)) list_move_tail(&u->link, &gc_candidates); } @@ -315,8 +315,8 @@ void unix_gc(void) BUG_ON(total_refs < inflight_refs); if (total_refs == inflight_refs) { list_move_tail(&u->link, &gc_candidates); - u->gc_candidate = 1; - u->gc_maybe_cycle = 1; + __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags); + __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags); } } @@ -344,7 +344,7 @@ void unix_gc(void) if (atomic_long_read(&u->inflight) > 0) { list_move_tail(&u->link, ¬_cycle_list); - u->gc_maybe_cycle = 0; + __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags); scan_children(&u->sk, inc_inflight_move_tail, NULL); } } @@ -356,7 +356,7 @@ void unix_gc(void) */ while (!list_empty(¬_cycle_list)) { u = list_entry(not_cycle_list.next, struct unix_sock, link); - u->gc_candidate = 0; + __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags); list_move_tail(&u->link, &gc_inflight_list); } _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
