Fix deadlock between epoll_wait and getdents

epoll_wait acquires EventPoll.listsMu (in EventPoll.ReadEvents) and
then calls Inotify.Readiness which tries to acquire Inotify.evMu.

getdents acquires Inotify.evMu (in Inotify.queueEvent) and then calls
readyCallback.Callback which tries to acquire EventPoll.listsMu.

The fix is to release Inotify.evMu before calling Queue.Notify. Queue
is thread-safe and doesn't require Inotify.evMu to be held.

Closes #121

PiperOrigin-RevId: 227066695
Change-Id: Id29364bb940d1727f33a5dff9a3c52f390c15761

1 files changed, 9 insertions, 4 deletions

diff --git a/pkg/sentry/fs/inotify.go b/pkg/sentry/fs/inotify.go
index f251df0d1..51ece5ed0 100644
--- a/pkg/sentry/fs/inotify.go
+++ b/pkg/sentry/fs/inotify.go
@@ -42,14 +42,14 @@ type Inotify struct {
 	// user, since we may aggressively reuse an id on S/R.
 	id uint64
 
-	// evMu *only* protects the event queue. We need a separate lock because
+	waiter.Queue `state:"nosave"`
+
+	// evMu *only* protects the events list. We need a separate lock because
 	// while queuing events, a watch needs to lock the event queue, and using mu
 	// for that would violate lock ordering since at that point the calling
 	// goroutine already holds Watch.target.Watches.mu.
 	evMu sync.Mutex `state:"nosave"`
 
-	waiter.Queue `state:"nosave"`
-
 	// A list of pending events for this inotify instance. Protected by evMu.
 	events ilist.List
 
@@ -212,7 +212,6 @@ func (i *Inotify) Ioctl(ctx context.Context, io usermem.IO, args arch.SyscallArg
 
 func (i *Inotify) queueEvent(ev *Event) {
 	i.evMu.Lock()
-	defer i.evMu.Unlock()
 
 	// Check if we should coalesce the event we're about to queue with the last
 	// one currently in the queue. Events are coalesced if they are identical.
@@ -221,11 +220,17 @@ func (i *Inotify) queueEvent(ev *Event) {
 			// "Coalesce" the two events by simply not queuing the new one. We
 			// don't need to raise a waiter.EventIn notification because no new
 			// data is available for reading.
+			i.evMu.Unlock()
 			return
 		}
 	}
 
 	i.events.PushBack(ev)
+
+	// Release mutex before notifying waiters because we don't control what they
+	// can do.
+	i.evMu.Unlock()
+
 	i.Queue.Notify(waiter.EventIn)
 }