userfaultfd: Use sequence counter with associated spinlock

A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.

Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.

If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-23-a.darwish@linutronix.de

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 52de29000c7e5623ca89d6d4202a6ffc112f9bd0..26e8b23594fb3d14a078e8f38b47a645229a7804 100644 (file)
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -61,7 +61,7 @@ struct userfaultfd_ctx {
        /* waitqueue head for events */
        wait_queue_head_t event_wqh;
        /* a refile sequence protected by fault_pending_wqh lock */
-       struct seqcount refile_seq;
+       seqcount_spinlock_t refile_seq;
        /* pseudo fd refcounting */
        refcount_t refcount;
        /* userfaultfd syscall flags */
@@ -1998,7 +1998,7 @@ static void init_once_userfaultfd_ctx(void *mem)
        init_waitqueue_head(&ctx->fault_wqh);
        init_waitqueue_head(&ctx->event_wqh);
        init_waitqueue_head(&ctx->fd_wqh);
-       seqcount_init(&ctx->refile_seq);
+       seqcount_spinlock_init(&ctx->refile_seq, &ctx->fault_pending_wqh.lock);
 }
 
 SYSCALL_DEFINE1(userfaultfd, int, flags)