srcu: Bit manipulation changes for additional reader flavor

Currently, there are only two flavors of readers, normal and NMI-safe.
Very straightforward state updates suffice to check for erroneous
mixing of reader flavors on a given srcu_struct structure.  This commit
upgrades the checking in preparation for the addition of light-weight
(as in memory-barrier-free) readers.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index f259dd83427211ee1fd64e7aa75eea6766c5fd13..9774bc500de51d8b8b686e5745124add03b86da9 100644 (file)
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
                if (IS_ENABLED(CONFIG_PROVE_RCU))
                        mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
        }
-       WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
+       WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
                  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
        return sum;
 }
@@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
        sdp = raw_cpu_ptr(ssp->sda);
        old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
        if (!old_reader_flavor_mask) {
-               WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
-               return;
+               old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);
+               if (!old_reader_flavor_mask)
+                       return;
        }
        WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
 }