[PATCH v3 1/3] fuse: fix missing barrier when checking io-uring readiness

fuse_block_alloc() reads fch->initialized and then fch->io_uring.
fch->io_uring is set before fch->initialized, ordered by the smp_wmb()
in fuse_chan_set_intialized(), but fuse_block_alloc() has no matching
read barrier between the two loads.

This may lead a CPU to observe fch->initialized=1 but fch->io_uring=0,
and skip the check that blocks request allocation until the io-uring
queues are ready. This can reintroduce the lock-order inversion deadlock
that commit 3393ff964e0f prevents.

Add an smp_rmb() barrier to pair with the smp_wmb() in
fuse_chan_set_initialized() to prevent this.

Fixes: 3393ff964e0f ("fuse: block request allocation until io-uring init is complete")
Cc: [email protected]
Reviewed-by: Bernd Schubert <[email protected]>
Signed-off-by: Joanne Koong <[email protected]>
---
 fs/fuse/dev.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 5763a7cd3b37..bd3b48c4fdff 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -85,7 +85,13 @@ void fuse_chan_set_initialized(struct fuse_chan *fch, struct fuse_chan_param *pa
 
 static bool fuse_block_alloc(struct fuse_chan *fch, bool for_background)
 {
-	return !fch->initialized || (for_background && fch->blocked) ||
+	if (!fch->initialized)
+		return true;
+
+	/* Pairs with smp_wmb() in fuse_chan_set_initialized() */
+	smp_rmb();
+
+	return (for_background && fch->blocked) ||
 	       (fch->io_uring && fch->connected && !fuse_uring_ready(fch));
 }
 
@@ -120,9 +126,6 @@ static struct fuse_req *fuse_get_req(struct fuse_chan *fch, bool for_background)
 			goto out;
 	}
 
-	/* Matches smp_wmb() in fuse_chan_set_initialized() */
-	smp_rmb();
-
 	err = -ENOTCONN;
 	if (!fch->connected)
 		goto out;
-- 
2.52.0