| Message ID | 1449856001-21177-1-git-send-email-will.deacon@arm.com |
|---|---|
| State | Accepted |
| Commit | b4b29f94856ad68329132c2306e9a114920643e3 |
| Headers | show |
On Wed, Dec 16, 2015 at 05:18:48PM -0800, David Daney wrote: > What is the status of this patch? It there a good likelihood that it will > make it into v4.4? > > If not, we should request that c55a6ffa6285 ("locking/osq: Relax atomic > semantics") be reverted for v4.4 I think Peter was going to send it to mingo as an urgent fix, but I've not seen anything from the tip-bot yet. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index d092a0c9c2d4..05a37857ab55 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -93,10 +93,12 @@ bool osq_lock(struct optimistic_spin_queue *lock) node->cpu = curr; /* - * ACQUIRE semantics, pairs with corresponding RELEASE - * in unlock() uncontended, or fastpath. + * We need both ACQUIRE (pairs with corresponding RELEASE in + * unlock() uncontended, or fastpath) and RELEASE (to publish + * the node fields we just initialised) semantics when updating + * the lock tail. */ - old = atomic_xchg_acquire(&lock->tail, curr); + old = atomic_xchg(&lock->tail, curr); if (old == OSQ_UNLOCKED_VAL) return true;
The Cavium guys reported a soft lockup on their arm64 machine, caused by c55a6ffa6285 ("locking/osq: Relax atomic semantics"): [ 68.909948] [<fffffe00000da03c>] mutex_optimistic_spin+0x9c/0x1d0 [ 68.909951] [<fffffe00006fe4b8>] __mutex_lock_slowpath+0x44/0x158 [ 68.909953] [<fffffe00006fe620>] mutex_lock+0x54/0x58 [ 68.909956] [<fffffe0000265efc>] kernfs_iop_permission+0x38/0x70 [ 68.909959] [<fffffe00001fbf50>] __inode_permission+0x88/0xd8 [ 68.909961] [<fffffe00001fbfd0>] inode_permission+0x30/0x6c [ 68.909964] [<fffffe00001fe26c>] link_path_walk+0x68/0x4d4 [ 68.909966] [<fffffe00001ffa14>] path_openat+0xb4/0x2bc [ 68.909968] [<fffffe000020123c>] do_filp_open+0x74/0xd0 [ 68.909971] [<fffffe00001f13e4>] do_sys_open+0x14c/0x228 [ 68.909973] [<fffffe00001f1544>] SyS_openat+0x3c/0x48 [ 68.909976] [<fffffe00000851f0>] el0_svc_naked+0x24/0x28 This is because in osq_lock we initialise the node for the current CPU: node->locked = 0; node->next = NULL; node->cpu = curr; and then publish the current CPU in the lock tail: old = atomic_xchg_acquire(&lock->tail, curr); Once the update to lock->tail is visible to another CPU, the node is then live and can be both read and updated by concurrent lockers. Unfortunately, the ACQUIRE semantics of the xchg operation mean that there is no guarantee the contents of the node will be visible before lock tail is updated. This can lead to lock corruption when, for example, a concurrent locker races to set the next field. Fixes: c55a6ffa6285 ("locking/osq: Relax atomic semantics"): Reported-by: David Daney <ddaney@caviumnetworks.com> Reported-by: Andrew Pinski <andrew.pinski@caviumnetworks.com> Tested-by: Andrew Pinski <andrew.pinski@caviumnetworks.com> Acked-by: Davidlohr Bueso <dave@stgolabs.net> Signed-off-by: Will Deacon <will.deacon@arm.com> --- kernel/locking/osq_lock.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/