errseq: eliminate special limitation for macro MAX_ERRNO

Current errseq implementation depends on a very special precondition that
macro MAX_ERRNO must be (2^n - 1).

Eliminate the limitation by

- redefining macro ERRSEQ_SHIFT
- defining a new macro ERRNO_MASK instead of MAX_ERRNO for errno mask.

There is no plan to change the value of MAX_ERRNO, but this makes the
implementation more generic and eliminates the BUILD_BUG_ON().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Zijun Hu <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/errseq.c

···
  */
 
 /* The low bits are designated for error code (max of MAX_ERRNO) */
-#define ERRSEQ_SHIFT		ilog2(MAX_ERRNO + 1)
+#define ERRSEQ_SHIFT		(ilog2(MAX_ERRNO) + 1)
 
 /* This bit is used as a flag to indicate whether the value has been seen */
 #define ERRSEQ_SEEN		(1 << ERRSEQ_SHIFT)
+
+/* Leverage macro ERRSEQ_SEEN to define errno mask macro here */
+#define ERRNO_MASK		(ERRSEQ_SEEN - 1)
 
 /* The lowest bit of the counter */
 #define ERRSEQ_CTR_INC		(1 << (ERRSEQ_SHIFT + 1))
···
 {
 	errseq_t cur, old;
 
-	/* MAX_ERRNO must be able to serve as a mask */
-	BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
 
 	/*
 	 * Ensure the error code actually fits where we want it to go. If it
···
 		errseq_t new;
 
 		/* Clear out error bits and set new error */
-		new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
+		new = (old & ~(ERRNO_MASK | ERRSEQ_SEEN)) | -err;
 
 		/* Only increment if someone has looked at it */
 		if (old & ERRSEQ_SEEN)
···
 
 	if (likely(cur == since))
 		return 0;
-	return -(cur & MAX_ERRNO);
+	return -(cur & ERRNO_MASK);
 }
 EXPORT_SYMBOL(errseq_check);
 
···
 		if (new != old)
 			cmpxchg(eseq, old, new);
 		*since = new;
-		err = -(new & MAX_ERRNO);
+		err = -(new & ERRNO_MASK);
 	}
 	return err;
 }